When EU law and national law clash, which one wins?

When EU law and national law clash, the Court of Justice says EU law wins, even over a member state's constitution. This principle is called the primacy of EU law. It was not written in the treaties: it was derived from an interpretation by the European Court of Justice, which ruled that European law has priority over any contravening national law, including a member state's constitution itself.

The case began with an electricity bill. In Costa v ENEL, Flaminio Costa, an Italian opposed to the nationalisation of energy companies, refused to pay his bill in protest. The Court ruled in 1964 that law stemming from the treaty could not be overridden by domestic legal provisions, however framed, without losing its character as Community law.

Not everyone accepts the whole package. Most national courts accept primacy, except for the part where European law outranks a member state's constitution, and national constitutional courts have reserved the right to review EU law against their own constitutions. Germany's court said in 1986 it would not review EU acts as long as EU law protected fundamental rights comparably, and in October 2021 Poland's Constitutional Tribunal issued a judgment challenging primacy in certain areas.

The EU's powers are also limited by the treaties through the principles of conferral, subsidiarity and proportionality. Under subsidiarity, outside its exclusive competence, the Union acts only if member states cannot sufficiently achieve the aims and they are better achieved at EU level.