Poland's Constitutional Tribunal had ruled that EU treaties were incompatible with Polish law, undermining the primacy of EU law.
The European Commission on Wednesday announced it was starting an infringement procedure against Poland for undermining EU law and the independence of its national judiciary.
The move comes amid an ongoing spat between Brussels and Warsaw over judicial independence and rule of law.
The EU Commission said it "has serious doubts on the independence and impartiality" of Poland's Constitutional Tribunal.
The Constitutional Tribunal, Poland's top court, had said that law out of Warsaw could take precedence over laws coming from Brussels.
The court had looked specifically at the compatibility of provisions from EU treaties — which are used by the European Commission to justify having a saw in the rule of law in member states — with Poland's constitution.
