European Union law is supreme over domestic legislation.

So European Union regulations and directives take precedence over domestic legislation.

But does this also mean the law contained within EU treaties (TEU, TFEU) is supreme over domestic legislation (eg. acts of parliament)?

In the Costa judgement this appears relevant:

It follows from all these observations that the law stemming from the treaty, an independent source of law, could not, because of its special and original nature, be overridden by domestic legal provisions, however framed, without being deprived of its character as community law and without the legal basis of the community itself being called into question

This implies EU treaty law is supreme?

  • 1
    Yes and no. It's complicated. There is a Spanish case establishing supremacy, but it is mitigated by the fact that lots of EU law is not self-executing and that there is a process for compelling nations to adopt EU directive compliant domestic law. Some EU legal instruments are self-executing and they are supreme, some are not.
    – ohwilleke
    Oct 8 '19 at 21:18
  • 3
    @ohwilleke this is further complicated by the situation arising when an EU member fails to meet its obligation to implement a (non-self-executing) directive by the deadline; in some cases, apparently, courts have ruled that the rights defined in the directive pertain nonetheless. Furthermore, the existence of both a directive and implementing national legislation can lead to conflicts, and the text of the directive (along with judicial precedent) will generally control the court's decision.
    – phoog
    Oct 8 '19 at 22:03
  • @ohwilleke that looks like an answer. Please answer in answers, not comments.
    – Dale M
    Oct 8 '19 at 22:55
  • @phoog that looks like an answer. Please answer in answers, not comments
    – Dale M
    Oct 8 '19 at 22:55
  • 1
    @DaleM it's a response to ohwilleke's comment. I am not well enough versed in the theory to write an answer; I only know what I've seen from following EU free movement law (a directive), especially as implemented in the UK. If nobody answers in a couple of days I'll try to do some research and write a proper answer.
    – phoog
    Oct 8 '19 at 23:13

Short answer

Costa was the first, but not the leading, case. Internationale Handelgesellschaft is. See below. I assume you're asking about EU law's supremacy over UK law, as you tagged [united-kingdom].

4.2 The scope and implications of the supremacy principle

Although Costa clearly established the principle of supremacy of EU law, questions remained about its precise scope and implications. Subsequent case law clarified two important points.
      First, in Simmenthal, the CJEU addressed the implications of the supremacy principle for national courts.88 The case arose from an Italian law requiring meat importers to pay for veterinarian checks at the national border. Although it was established that this was contrary to directly effective EU law concerning the free movement of goods, the Italian government argued that domestic courts could not order it to repay the fees until and unless the Italian Constitutional Court annulled the law in question. This view did not find favour with the CJEU, which took the opportunity to spell out the practical consequences of the supremacy doctrine for domestic courts faced with incompatible national laws. The CJEU said it was the task of national courts to protect the rights conferred upon individuals by EU law.89 Domestic courts therefore had to ‘apply [Union] law in its entirety’, disregarding ‘any provision of national law’—whenever enacted—‘which may conflict with it’, and ignoring any national rules that, if applied, would compromise domestic courts’ capacity to do the foregoing things.90
      Second, in Internationale Handelsgesellschaft, the CJEU held that the principles laid down in Simmenthal applied to all types of national law, including provisions of member states’ constitutions, and including provisions in such constitutions concerning the protection of human rights.91 It was therefore the duty of national courts to disapply national constitutional human rights guarantees to the extent that EU law was inconsistent with them. Unsurprisingly, this conclusion provoked a good deal of disquiet, but the Court was careful to say that such situations should arise rarely (if at all). This was because EU law itself recognised the fundamental rights common to the constitutional traditions of member states. As a result, the Union was bound by those rights and powerless to legislate or otherwise act in breach of them.92 It would therefore rarely, if ever, be the case that a national court was forced to choose between a national human rights law and a valid EU provision.

88 Case 106/77 Amministrazione delle Finanze dello Stato v Simmenthal SpA [1978] ECR 629.
89 Simmenthal, [16]. 90 Simmenthal, [17]–[22].
91 Case 11/70 Internationale Handelsgesellschaft mbH v Einfuhr- und Vorratsstelle für Getreide und Futtermittel [1970] ECR 1125.
92 Case 4/73 J Nold Kohlen- und Baustoffgrosshandlung v Commission of the European Communities [1975] ECR 985. See further Tridimas, General Principles of EU Law (Oxford 2005), ch 7.

Mark Elliott, Robert Thomas. Public Law 2020 4 edn. pp 380-1.

The validity of Union law could thus not be affected – even by the most fundamental norms within the Member States. The Court’s vision of the supremacy of European law over national law was an absolute one: ‘The whole of [European] law prevails over the whole of national law.’15

15 R. Kovar, ‘The Relationship between Community Law and National Law’, in EC Commission (ed.), Thirty Years of Community Law (EC Commission, 1981), 109 at 112–13

Robert Schutze, European Union Law 2018 edn, p 124.

Long answer

(B) Ambit: Supremacy Applicable Against All National Law

While the conceptual basis for the supremacy of EU law was set out in Costa [that you mooted in your post], the ambit of the principle became clearer in later decisions. In the following case, the Court ruled that the legal status of a conflicting national measure was not relevant to whether EU law should take precedence.3 Not even a fundamental rule of national constitutional law could be invoked to challenge the supremacy of a directly applicable EU law.

      This ruling gave rise to a potentially serious conflict in the relationship between the German Federal Constitutional Court and the ECJ. While the latter has sought to avoid a direct constitutional conflict with a national court,4 it has never retreated from its claims.

Case 11/70 Internationale Handelsgesellschaft mbH v Einfuhr- und Vorratsstelle für Getreide und Futtermittel [1970] ECR 1125

The applicant argued that a Community regulation under which a deposit would be forfeited if the goods were not exported within the period of time set was contrary to principles of national constitutional law, including freedom of action and of disposition, economic liberty, and proportionality.


  1. Recourse to the legal rules or concepts of national law in order to judge the validity of measures adopted by the institutions of the Community would have an adverse effect on the uniformity and efficacy of Community law. The validity of such measures can only be judged in the light of Community law. In fact, the law stemming from the Treaty, an independent source of law, cannot because of its very nature be overridden by rules of national law, however framed, without being deprived of its character as Community law and without the legal basis of the Community itself being called into question. Therefore the validity of a Community measure or its effect within a Member State cannot be affected by allegations that it runs counter to either fundamental rights as formulated by the constitution of that State or the principles of a national constitutional structure.

      The Court faced the opposite kind of argument in Ciola, where the Austrian Government argued that the principle of primacy should not automatically apply ‘to specific individual administrative acts’.5 The Court dismissed this argument, reaffirming that any provision of national law that conflicted with directly effective EU law should not be applied. Thus, the principle of primacy is required whenever directly effective EU law is concerned, and regardless of whether fundamental national constitutional norms, or minor administrative acts, are at issue. The Court, however, qualified Ciola by admitting that, under specific circumstances, supremacy needs to be accommodated with domestic limitations of the period of time during which administrative acts may be repealed or judicially contested.6

3 See also Case C–473/93 Commission v Luxembourg [1996] ECR I–3207, [38]; Case C–273/15 ZS ‘Ezernieki’ v Lauku atbalsta dienests EU:C:2016:364, [53]; Case C–516/17 Spiegel EU:C:2019:625, [19]; Case C–476/17 Pelham EU:C:2019:624, [78].
4 See, eg, Case C–446/98 Fazenda Pública v Câmara [2000] ECR I–11435, [36]–[38].
5 Case C–224/97 Ciola v Land Vorarlberg [1999] ECR I–2517, [24].
6 Case C–453/00 Kühne & Heitz [2004] ECR I–837; Case C–2/06 Willy Kempter AG [2008] ECR I–411. See Ch 14 for discussion.

Op cit, pp 316-7.

Why must I know which Member State you're asking about, to best answer your question? Because

There is a continuing tension between national accounts of EU law and the CJEU’s account. Constitutional conflicts continue to arise in specific cases, and it remains for national courts to resolve cases arising before them involving a conflict between EU and national law.46 Reasons of space preclude detailed consideration of all Member States. The ensuing analysis therefore focuses on five Member States, Germany, Italy, France, Poland, and the Czech Republic, thereby including an admixture of original signatories and states that joined post-2004.

46 M Claes, The National Courts’ Mandate in the European Constitution (Hart, 2006); A-M Slaughter, A Stone Sweet, and J Weiler (eds), The ECJ and National Courts: Doctrine and Jurisprudence (Hart, 1998); K Alter, Establishing the Supremacy of European Law: The Making of an International Rule of Law in Europe (Oxford University Press, 2001); N Walker (ed), Sovereignty in Transition (Hart, 2003).

Here's how Supremacy of EU Law works for UK Post-Brexit. I quote some salient paras. from pp 363-4.

i. The principle of the supremacy of EU law in relation to the UK is dealt with in sections 5 and 6 of the European Union (Withdrawal) Act 2018, EUWA, as amended by the European Union (Withdrawal Agreement) Act 2020. The position is as follows.

ii. First, section 5(1) EUWA provides that the principle of the supremacy of EU law does not apply to any enactment or rule of law passed or made on or after the end of the implementation period, which is 31 December 2020, known in the legislative jargon as IP completion day. This is in accord with the logic of UK withdrawal from the EU.

iii. Secondly, section 5(2) EUWA then provides that ‘accordingly, the principle of the supremacy of EU law continues to apply on or after IP completion day so far as relevant to the interpretation, disapplication, or quashing of any enactment or rule of law passed or made before IP completion day’. It means that if there is a conflict between pre-IP completion day domestic legislation and retained EU law, the latter takes precedence. It also means that pre-IP completion day domestic law should be interpreted, as far as possible, in accordance with retained EU law. The policy rationale underlying section 5(2) is legal certainty: prior to IP completion day, EU law had supremacy over domestic law, and it was felt that this should therefore continue.

vii. Thirdly, section 5(1) EUWA is further qualified by section 5(3). It provides that section 5(1) ‘does not prevent the principle of the supremacy of EU law from applying to a modification made on or after IP completion day of any enactment or rule of law passed or made before IP completion day if the application of the principle is consistent with the intention of the modification’. The application of this qualification can clearly be contestable, and the issue may well have to be decided by the UK courts in the light of the evidence that is available from Parliament, which is admissible in court.

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