The principle of supremacy of EU law
53 A key principle of EU law is that EU law is supreme, which means that it has the status of a superior source of law within the EU’s member states. Domestic laws must give way and be disapplied by domestic courts if they are found to be inconsistent with EU law. Sometimes referred to as the principle of the primacy of EU law, this core rule of EU law was established in the case law of the CJEU before the accession of the UK to the European Communities. This is made clear, for example, in the judgment of the CJEU in Costa 1:
"By contrast with ordinary international treaties, the EEC Treaty has created its own legal system which, on the entry into force of the Treaty, became an integral part of the legal systems of the Member States and which their courts are bound to apply.
By creating a community of unlimited duration, having its own institutions, its own personality, its own legal capacity and capacity of representation on the international plane and, more particularly, real powers stemming from a limitation of sovereignty or a transfer of powers from the States to the community, the Member States have limited their sovereign rights, albeit within limited fields, and have thus created a body of law which binds both their nationals and themselves."
1 Costa v ENEL  ECR 585 (Case 6/64)