National Limits, European Claims: The Ultra Vires Conflict in EU Constitutional Law (part 1)

 

Dr Benedikt Riedl, Mjur (Oxford),  postdoctoral researcher and academic assistant at the Chair of Public Law and Legal Philosophy (Prof Dr Peter M. Huber, former Justice of the Federal Constitutional Court and former Minister of the Interior) at LMU Munich.

Photo credit: Luxofluxo, via Wikimedia Commons

 

Who has the final say on where the European Union’s competences end? This question lies at the core of one of the most debated issues in EU constitutional law: ultra vires review. Long seen as an instrument of national resistance to the primacy of EU law – especially in the case law of the German Federal Constitutional Court (GCC) – ultra vires review is often portrayed as an outlier. But that view misses the broader reality. Across the EU, many constitutional courts claim a residual authority to decide whether EU institutions have overstepped their conferred powers.

This blogpost – the first of two parts – re-examines the legal nature of ultra vires review. In 83 theses, it argues that ultra vires review is not just a defensive reaction, but a structural element of the European legal order. It operates within a pluralist and composite system in which both the CJEU and national constitutional courts have essential and complementary roles. Part One begins by tracing the national doctrines that support ultra vires review, showing how firmly it is anchored in the constitutional traditions of many Member States (I.). It then turns to the institutional tensions between national courts and the CJEU, focusing on the latter’s claim to exclusive authority over the scope and limits of Union law (II.).

I. Ultra Vires Review as a European Practice: Constitutional Courts and the Limits of Integration

1 Ultra vires review enables Member States’ supreme and constitutional courts to (indirectly) assess whether EU institutions have acted beyond the powers conferred upon them by the EU Treaties. These courts justify their role by emphasizing that the EU operates on the basis of limited and conferred competences (the principle of conferral) and that national constitutions impose boundaries on the application of EU law. They argue that any exercise of power not explicitly transferred to the EU by the Member States constitutes an infringement on national – and ultimately popular – sovereignty. Accordingly, ultra vires review serves as a democratic safeguard, protecting the constitutional autonomy of Member States.

2 The CJEU, by contrast, regards the EU legal order as autonomous and independent of Member States’ constitutions. It views ultra vires review as an infringement on its exclusive authority to declare EU acts invalid. The Court argues that such reviews undermine the uniformity and effectiveness of EU law, weaken the integrity of the EU judicial system, and disrupt the principle of equality among Member States. In its view, recognising ultra vires review would ultimately threaten the effectiveness of the European project as a whole.

3 Ultra vires review by national constitutional courts can potentially conflict with the CJEU’s jurisdiction under Article 19(1) EU. However, the core issue in the ultra vires debate is clear: the EU is bound by the principle of conferral, as set out in Article 5(1) and (2) TEU. What remains unresolved is the procedural question: Which court has the final authority to decide whether EU institutions, bodies, offices, or agencies have exceeded their competences? Quis iudicabit? The EU Treaties contain no explicit conflict-of-law provision to resolve judicial disputes over ultimate decision-making power in matters of competence.

4 This thesis examines ultra vires review as a legal institution within EU law and explores how a common ultra vires procedure could be structured in accordance with the EU Treaties. To this end, it compares the CJEU’s interpretation of EU law with the fundamental principles enshrined in the Treaties, and analyses the extent to which the perspectives of national constitutional courts have been integrated into EU legal doctrine. The central question is whether ultra vires review can be considered a legal concept within EU law, and to what extent constitutional courts should be involved in a collective ultra vires procedure. The study is based on a comprehensive analysis of the constitutional and EU law foundations of ultra vires review. Its aim is to determine whether the power of constitutional courts to declare EU measures ultra vires and inapplicable in their domestic legal systems constitutes a general principle of EU law.

5 In most EU Member States, constitutional reservations concerning the application of EU law exist. Courts in Germany, Denmark, France, the Czech Republic, and Poland have already declared CJEU judgments inapplicable on the grounds that they constituted ultra vires acts, i.e. acts exceeding the EU’s conferred competences. Moreover, constitutional and supreme courts in many other Member States have adopted similar reservations, serving comparable functions in safeguarding national constitutional limits within the EU legal framework.

6 The GCC’s approach to ultra vires review is based on Article 23(1) sentences 2 and 3, in conjunction with Article 20(1) and (2), and Article 79(3) of the German Basic Law.  By applying ultra vires review, the GCC ensures that EU law remains democratically legitimate in Germany, thereby adhering to the so-called “integration programme” (Integrationsprogramm) defined by the parliaments’ acts of assent. This review is triggered only in cases of manifest and structurally significant overreach of competences and is handled exclusively by the GCC, with a mandatory referral to the CJEU. The GCC respects the CJEU’s interpretation of competence limits and confines itself to reviewing the methodological soundness of that interpretation. The core rationale behind ultra vires review – the protection of popular sovereignty – is persuasive. The concept of “responsibility for integration” (Integrationsverantwortung) illustrates how the review mechanism serves to preserve democratic legitimacy. This responsibility, entrusted to German constitutional bodies, compensates for the partial loss of German citizens’ political self-determination resulting from the transfer of sovereign powers to the EU. While ultra vires review does not interfere with the substance of EU decisions, it ensures democratic openness by exposing breaches of legitimacy and blurred lines of accountability. In doing so, it partially offsets the absence of a fully developed European public sphere, enabling citizens to retain as much political self-determination as possible. As a last resort, it prevents citizens from being excluded from fundamental political decisions affecting their community.

7 In Denmark, the Højesteret bases its ultra vires review on the principle of legal certainty as articulated in § 20(1) of the Danish Constitution. This provision allows for the transfer of sovereign powers to international organisations and the EU, but only “to a specified extent.” The court’s justification is formal yet consistent, as linked directly to the principle of conferral. The scope of EU law's primacy is defined by the national act of assent; if the EU extends its competences beyond what is covered by that act, it is considered a violation of § 20, which falls under the scrutiny of the Højesteret. However, the standard of review and the limits of ultra vires control compatible with EU law remain vaguely defined in Danish case law. In the Carlsen v. Rasmussen judgment, the application of ultra vires review appeared to be confined to particularly exceptional circumstances and was closely tied to the outcome of the CJEU’s preliminary ruling. By contrast, in the Ajos case, the Højesteret upheld an ultra vires finding without elaborating on any such restrictive conditions, leaving the precise contours of the doctrine unclear.

8 In France, a flexible balancing of principles allows both the Conseil constitutionnel and the Conseil d'État to indirectly review the principle of conferral, alongside questions of constitutional identity. Although both courts formally recognise the primacy of EU law, they continue to treat the French Constitution as the supreme legal authority, in line with Article 88-1 of the French Constitution. To reconcile these positions, the courts have developed two review mechanisms: the identity test (employed by the Conseil constitutionnel) and the equivalence test (applied by the Conseil d’État). The core objective of both is to ensure that EU law maintains a level of protection equivalent to that guaranteed by the French Constitution. Within this broad, principle-based balancing approach, substantive legal issues can be linked to the preliminary question of competence, as demonstrated in the French Data Network decision by the Conseil d’État. Functionally, the equivalence test renders a separate ultra vires review redundant, thereby allowing for greater flexibility in individual judgments. However, the fact that the standard of review remains implicit – particularly where constitutionally protected interests are vaguely defined – can lead to diminished legal certainty.

9 The independent Polish Constitutional Tribunal – prior to the judicial reforms introduced by the PiS government – based its ultra vires review on Article 8(1) in conjunction with Article 90(1) of the Polish Constitution. According to Article 8(1), the Polish Constitution remains the supreme law of the Republic of Poland. Article 90(1) permits the transfer of certain competences to the EU, granting them priority of application. However, any subsequent expansion of competences must still comply with the requirements of Article 90. For the Polish Constitutional Tribunal, the procedural review mechanism was regarded as a necessary safeguard for the substantive principle of limited conferred powers. Although the procedure and standard of review were never clearly defined, the ultra vires review remained confined to exceptional cases involving a significant and manifest overreach of competences, and required a prior referral to the CJEU. In this respect, the Tribunal’s case law was largely modelled on that of the GCC. However, the more recent ultra vires ruling in case K 3/21, issued by a Constitutional Court now widely viewed as politically aligned, has been criticised as politically motivated and stands in clear contradiction to fundamental principles of EU law.

10 The Czech Constitutional Court bases its ultra vires review on Article 10a, in conjunction with Article 9(2) and Article 1(1) of the Czech Constitution. Under Article 10a, the validity of EU law in the Czech legal system depends on the national acts of assent. The ultra vires review serves to protect democratic self-determination and state sovereignty, both of which are unalterably safeguarded under Article 9(2) in conjunction with Article 1(1). The Court’s justification for ultra vires review therefore goes beyond the formal principle of conferral, encompassing broader substantive constitutional values such as sovereignty and self-determination. However, the legal standard governing Czech ultra vires review remains underdeveloped. Given that the Constitutional Court recognizes the CJEU’s exclusive jurisdiction to declare EU acts invalid, it confines its own review to “very exceptional” cases as a measure of last resort. In its ultra vires decision in Holubec, however, the Court did not clarify the criteria for such exceptional cases and declined to refer the matter to the CJEU, thereby failing to adequately observe the duty of cooperation under EU law.

11 The Hungarian Constitutional Court’s judgment No. 22/2016 (XII. 5.) on constitutional identity and ultra vires review – alongside the Polish judgment K 3/21 – clearly illustrates the instrumentalisation of these control mechanisms. Rather than serving as an independent instrument of constitutional oversight, they function primarily as political gestures by captured courts in favour of their respective governments. The Hungarian ruling lacks dogmatic coherence and is not grounded in a legal tradition compatible with EU law. Instead, it relies on a superficial and overly modular legal comparison that lacks both depth and consistency. The most serious flaw, however, is the absence of a meaningful commitment to dialogue and cooperation with the CJEU. While the judgment formally invokes a willingness to engage, it fails to substantiate this claim with any concrete standards compatible with European legal principles.

12 In an obiter dictum to its judgment No. 62/2016, the Belgian Constitutional Court addressed the constitutional limits of the EU legal order. It based its constitutional identity reservation in Article 34 of the Belgian Constitution, which permits the transfer of sovereign powers to international organisations and the EU, but only to a limited extent. Article 34 also implies that the validity of EU law within the Belgian legal order is conditional upon a constitutional review of its boundaries. The provision does not grant EU institutions a carte blanche, i.e. a general or unlimited authorisation to act. The identity review was central to the Court’s reasoning. At the same time, an ultra vires restriction is implicitly embedded in the constitutional logic of conferral through the national delegation of powers under Article 34 – even if not expressly articulated as such.  

13 The Estonian State Court defines the sovereignty of the Estonian people and state as establishing the constitutional boundaries of the EU legal order. While the primacy of EU law is acknowledged, any treaty amendments or expansions of competences must be legitimised by the Estonian parliament in order to have domestic legal effect. The Court thus assesses whether the delegation of competences results in a constitutional infringement and whether such delegation has been sufficiently legitimised through parliamentary approval. Since neither the autonomy nor the absolute primacy of EU law is recognised and the validity of EU law is grounded in the Estonian Constitution, there exists a clear constitutional basis for the potential future development of an ultra vires review.

14 In Ireland, the primacy of EU law must not override the country’s constitutional identity, which includes essential fundamental rights – such as the right to life – as well as legislative and judicial sovereignty, the principle of democracy, and, most importantly, the sovereignty of the people. This constitutional identity, as anchored in Article 5 of the Irish Constitution, imposes clear limits on integration into international organisations, including the EU. However, the Irish Constitution and the Irish Supreme Court adopt a distinct approach to safeguarding popular sovereignty: they place primary emphasis on maximising democratic legitimacy at the point of transferring sovereign powers. As a result, any such transfer to the EU requires a constitutional amendment, which in turn must be approved by referendum.

15 The Italian Constitutional Court bases its constitutional reservation (controlimiti) on the principle of Italian sovereignty, which – although it can be limited by the national legislature – cannot be fully relinquished. The Court examines whether such limitations infringe upon fundamental rights, essential constitutional principles, or the elements of constitutional identity. Since the primacy of EU law is viewed as a consequence of the domestic limitation of sovereignty under Article 11 of the Italian Constitution, the ultra vires reservation is implicitly embedded in the Constitution. This provision enables the Corte Costituzionale to review national acts of assent for their compatibility with the fundamental values of the Italian Constitution. The concept of ultra vires review is most clearly articulated in the Fragd decision, where the Court not only scrutinised the CJEU’s interpretation of EU law but also questioned whether the CJEU had acted beyond its conferred competences. However, due to the Court’s broad understanding of identity review, ultra vires review has been functionally sidelined, operating instead as a supplementary mechanism of constitutional oversight.

16 The Croatian Constitutional Court has indicated in several obiter dicta that it does not recognise an absolute or unlimited primacy of EU law. Instead, it affirms that constitutional limits apply to the primacy of EU law within the Croatian legal system.

17 The Latvian Constitution provides the doctrinal foundations for a potential future ultra vires review. Article 68 serves as the constitutional basis for the primacy of EU law within the Latvian legal system. According to the case law of the Latvian Constitutional Court, the ratification of the EU treaties is compatible with the Constitution only insofar it does not undermine fundamental constitutional values, particularly the principle of popular sovereignty. The Court has affirmed that compliance with these core values remains subject to judicial review, even in the context of European integration.

18 According to the judgment of the Lithuanian Constitutional Court, the primacy of EU law is derived from Section 2 of the Constitutional Law and is therefore rooted in domestic law. In exceptional cases, where an EU legal provision is found to conflict with the Lithuanian Constitution, that provision would have no effect within the Lithuanian legal order. However, the case law on the relationship between EU law and the Lithuanian Constitution remains underdeveloped and requires further clarification, particularly concerning its alignment with EU law principles. From a constitutional perspective, an ultra vires review would be legally permissible.

19 In Spain, the transfer of sovereign powers to the EU is governed by Article 93 of the Spanish Constitution. The Spanish Constitutional Court distinguishes between the formal supremacy (supremacía) of Spanish constitutional law and the primacy (primacía) of EU law, not conceiving them in a hierarchical relationship but rather as separate spheres of application. The transfer of competences under Article 93 imposes constitutional limits on European integration, requiring EU law to remain compatible with the fundamental principles of Spain’s social and democratic constitutional state. The Court explicitly reserves a constitutionally entrenched power of final decision, derived from Article 93, which it may invoke in exceptional cases to safeguard the sovereignty of the Spanish people and the supremacy of the Spanish Constitution. Although the Court has acknowledged the hypothetical possibility that EU primary law could exceed conferred competences – potentially infringing on Spanish sovereignty and the constitutional core – it has not explicitly established an ultra vires review as a distinct legal instrument.

20 In Greece, the primacy of EU law is grounded in Article 28(2) and (3) of the Greek Constitution. These provisions also establish limits to the primacy of EU law, particularly concerning human rights, the foundations of the democratic state, and national sovereignty.

21 In Portugal, according to Article 8(4) of the Portuguese Constitution, EU law is applicable within the national legal system only insofar as it respects the principles of the democratic constitutional state. The Portuguese Constitutional Court has ruled that in a multi-level constitutional system, where multiple legal orders coexist, legal norms that have domestic effect cannot override the Constitution. Accordingly, EU law must respect the national identity of Member States, including Portugal.

22 In Romania, the transfer of certain powers to the EU occurs through the national act of assent, in conjunction with Article 148(1) of the Romanian Constitution. The Romanian Constitutional Court derives the primacy of the national Constitution over EU law from Article 11(3) of the Constitution. Recent rulings suggest that the Court is strategically asserting the supremacy of the Romanian Constitution in relation to EU law, positioning it as a counterweight within the national legal order.

23 In Luxembourg and Slovakia, the relationship between EU law and national constitutional law remains unclear. In Malta, Article 6 of the Maltese Constitution explicitly designates the Constitution as the highest source of law within the national legal system. A similar position exists in Bulgaria, where Article 5(1) of the Bulgarian Constitution states that no other laws may contradict the Constitution. The Finnish Parliamentary Committee on Constitutional Affairs has recently begun to address both the ultra vires issue and the protection of core constitutional principles. In Slovenia, the primacy of EU law is based on the national act of assent in conjunction with Article 3a(1) of the Slovenian Constitution, making an ultra vires challenge possible in the future. In Sweden, courts generally acknowledge the primacy of EU law, though constitutional reservations are not ruled out. In Austria, the transfer of sovereign powers to the EU is carried out through the parliament’s act of assent and is subject to Article 44(3) of the Austrian Constitution, which offers special protection to the fundamental principles of the Federal Constitution. The Austrian Constitutional Court has recently implicitly recognised the principle of democracy as part of Austria’s constitutional identity. Only in the Netherlands and Cyprus, the absolute primacy of EU law is generally assumed, though even there, its validity is still grounded in national acts of assent by the Member State parliaments. Finally, in the United Kingdom, which formally left the EU on 31 January 2020, the Supreme Court continues to apply an ultra vires doctrine, drawing on the jurisprudence of the GCC.

24 In summary, ultra vires review is not a concept limited to Germany; it is explicitly recognised in the case law of six EU Member States: Denmark, Germany, the Czech Republic, France, Poland, and Hungary. Furthermore, constitutional rulings and legal foundations in Belgium, Estonia, Italy, and Latvia also acknowledge the possibility of invoking ultra vires arguments. Additionally, Croatia, Lithuania, Greece, Romania, Slovenia, and Spain accept at least the doctrinal basis for a potential ultra vires review. A significantly higher number of Member States conduct some form of identity review or apply comparable constitutional safeguards. In total, 18 Member States – a clear majority – explicitly reject the notion of absolute primacy of EU law. This number is particularly notable given that in several of the remaining Member States, the scope of EU law's primacy has yet to be addressed, often due to the absence of constitutional jurisdiction.

25 The CJEU’s justification of the primacy of EU law based on its autonomy concept is not accepted by any national constitutional court. Instead, the validity and precedence of EU law are understood to derive from the acts of assent passed by the parliaments of the Member States, which authorize the application of EU law within their respective legal systems.

26 On this basis, the clear majority of constitutional and supreme courts in the Member States do not accept the absolute primacy of EU law over their national constitutions, but instead assert constitutional reservations. In most EU countries, it is standard practice to assess EU treaties in light of national constitutional law. Many constitutional courts extend these reservations to secondary EU law, typically through one of three types of review: the Solange review (equivalent protection doctrine), identity review, and ultra vires review. These mechanisms serve to protect various dimensions of constitutional identity – whether it involves safeguarding fundamental rights, preserving core substantive constitutional principles, or defending national sovereignty and the integration programme.

 

II. The CJEU’s Constitutional Vision: Autonomy, Primacy, and the Rejection of Ultra Vires Review

Taken together, the constitutional jurisprudence of the Member States reveals a clear trend: ultra vires review is not an isolated German construction, but a widespread legal safeguard anchored in the principle of conferred powers. At least eighteen Member States explicitly reject the idea of absolute primacy and maintain constitutional reservations, particularly where Union acts threaten to undermine the democratic legitimacy or constitutional identity of the state. The CJEU, by contrast, categorically rejects the legitimacy of such review. This is not just a matter of institutional competition – it reflects a deeper constitutional conflict. The CJEU views EU law as an autonomous legal order that does not depend on national constitutions for its validity. Based on this view, it claims absolute primacy of EU law over all national norms, including constitutional provisions. However, this conception is neither codified in the Treaties nor accepted by most Member States. It rests on a dogmatic assumption: that the EU legal order is self-originating. Nevertheless, the CJEU relies on it to support a far-reaching claim to institutional supremacy. Over time, the CJEU has translated its concept of autonomy into a hierarchical constitutional structure. It does so not only through substantive doctrines like absolute primacy, but also via procedural mechanisms that establish the CJEU as the final judge of its own jurisdiction. The result is a constitutional logic in which Member States lose both the ability to define the limits of EU competences and to protect their constitutional identity.

 

1 The CJEU rejects ultra vires review based on its interpretation of the primacy of EU law as being absolute. However, this concept of primacy has never been explicitly codified in the EU treaties. The Protocol on the Application of the Principles of Subsidiarity and Proportionality, annexed to the Treaty of Amsterdam, merely alluded to the primacy of EU law. A formal codification was intended in the Treaty establishing a Constitution for Europe, yet the ratification of this treaty ultimately failed. The subsequent Treaty of Lisbon did not include a binding provision on primacy either. Instead, only a legally non-binding interpretative declaration was attached, which referred back to the CJEU’s established case law on the matter. Similarly, Article 20 of the Unified Patent Court Agreement (UPCA) does not contain any provision explicitly stipulating the primacy of EU law.

2 According to established CJEU case law, all forms of EU law take precedence over national law, including national constitutional provisions and the jurisprudence of constitutional courts. The CJEU seeks to establish an absolute hierarchy, placing EU law above national constitutional orders. This position, however, directly conflicts with the Member States’ understanding of constitutional limits, which are considered inviolable. Since ultra vires review is grounded in the constitutional identity of the Member States, this creates a central point of tension between the CJEU’s approach and the constitutional reservations upheld by national constitutional courts.

3 The CJEU’s central argument for asserting absolute primacy is the autonomy of the EU legal order – the idea that EU law does not derive from the legal systems of the Member States. This key concept of autonomy not only forms the basis for the absolute primacy of EU law but has also been invoked in recent CJEU judgements to justify a comprehensive monopoly on jurisdiction within the European constitutional court network. Its claim to institutional supremacy has evolved into a specific application of the autonomy principle and is used to reinforce and safeguard the absolute primacy of EU law.

4 In contrast to other legal systems, the CJEU has institutionally expanded its concept of autonomy, aiming to secure itself a monopoly over jurisdiction and competence vis-à-vis national constitutional and supreme courts. Through infringement proceedings and by extending state liability to cases where national supreme courts violate EU law, the CJEU has structurally integrated and subordinated national courts within the broader European judicial system. This development marks a clear shift from a cooperative relationship among courts of equal standing to a hierarchical judicial structure with the CJEU at the top. Since the Portuguese Judges ruling, the CJEU has also gained the ability to issue detailed substantive criteria regarding the organisation of national judicial systems. Looking ahead, this could even extend to national courts being required to disregard ultra vires rulings issued by their own constitutional courts – invoking the overarching principle of the rule of law as justification.

5 At the intersection with international law, the CJEU has progressively strengthened its institutional position to safeguard the uniform application of EU law, including in relation to international courts. While this analysis does not focus on the EU’s external legal relationships, it is nonetheless clear that the CJEU understands the autonomy of EU law not only as independence from Member States, but also as a means of protection from external judicial interference. In institutional terms, the concept of autonomy thus translates into a reluctance to cooperate with “competing” judicial actors within a multi-level international legal order. The CJEU positions itself as the central authority in the European legal order, thereby establishing a hierarchy in which all legal systems interacting with EU law – and all courts whose jurisdiction overlaps with that of the CJEU, including the ECHR, arbitration tribunals, and national constitutional courts – are subordinated to EU law.

6 In the procedural and institutional conflict surrounding ultra vires review, the CJEU underscores the link between the autonomy of EU law, its absolute primacy, and the question of competences. It treats the scope of EU law's validity – including its limits – as falling within the domain of its absolute primacy. Consequently, national courts are never permitted to declare EU legal acts invalid, even if the alleged invalidity stems from an overreach of competences. Relying on this understanding of primacy and autonomy, the CJEU claims exclusive authority to determine both the validity and outer boundaries of EU law – effectively asserting judicial competence over competences. Thus, the CJEU has recently rejected any form of constitutional reservation by Member States in this regard.

7 Based on this interpretation, the concept of autonomy becomes a code for the CJEU’s exclusive jurisdiction over all matters concerning EU law. Any court addressing issues that affect EU law must defer to the CJEU’s final judgment. This absolute monopoly on judicial authority effectively excludes the possibility of cooperation within the network of constitutional courts – even with regard to questions of jurisdiction. The CJEU disregards the views of the Member States, treating its notion of autonomy as overriding even standards set out in primary EU law. Institutionally, this results in the absolutisation of the Court’s role. Under the guise of autonomy, the CJEU claims the power to continuously expand the boundaries of its jurisdiction, both within the EU and in relation to external actors.

 

Taken together, the conflict over ultra vires review reveals a deeper constitutional tension within the EU legal order: while the CJEU seeks to consolidate a uniform system based on the principle of absolute primacy, most national constitutional and supreme courts insist on the conditional nature of EU law’s validity, grounded in their domestic constitutional orders. This divergence cannot be resolved by institutional assertion alone. It requires a principled legal response that reflects the composite structure of the Union. Ultra vires review offers such a response. Far from being a disruptive anomaly, it is a functional safeguard embedded in the constitutional practice of a clear majority of Member States. It reflects the principle of conferral under Article 5(1) and (2) TEU and embodies the idea that competences remain with the Member States unless expressly transferred. The absence of a conflict-resolution mechanism in the Treaties does not negate the legitimacy of this review; rather, it implies the need for constitutional safeguards that ensure the accountability of EU institutions within the limits of conferred powers. In this light, ultra vires review is not merely a national instrument of resistance, but an emerging general principle of EU law. It serves to stabilise the Union’s legal architecture where the Treaties are silent, and it institutionalises constitutional pluralism without dissolving legal unity. Properly understood, it expresses a shared constitutional responsibility for the integrity of the European legal order. The second part of this blogpost examines whether – and how – ultra vires review can be understood as structured, treaty-compatible mechanism that operates as a joint procedure between national courts and the CJEU.