Brussels has today begun legal action against Poland over a row about the primacy of EU law in the country.
In October, Poland's Constitutional Tribunal ruled the country's EU membership did not give the bloc's courts supreme legal authority.
The ruling sparked anger in Brussels and further damaged Poland's already troubled relationship with EU chiefs.
On Wednesday, the European Commission said it had "serious concerns" over the ruling and said it had begun an "infringement procedure."
It said the ruling was in breach of EU treaties and denied individuals before Polish courts "the right to effective judicial protection."
"Finally, the Commission has serious doubts on the independence and impartiality of the Constitutional Tribunal," the statement added.
Poland has two months to reply.
The Polish government has, in recent years, passed a series of laws overhauling the country's judiciary, allowing the government to sack judges opposed to reforms, lowering the retirement age of Supreme Court justices and creating a controversial chamber of the Supreme Court devoted to disciplining judges.
The Polish government maintains the reforms were necessary to streamline the judiciary and eliminate the remains of the communist regime. Critics say the moves have effectively stripped the judiciary of its independence, a pillar of any democratic system.
The EU's top court, the European Court of Justice (ECJ), issued an injunction in July to suspend both the disciplinary chamber and the effects of the decisions already taken on the lifting of judicial immunity.
Wednesday's ruling found that "Poland has failed to take the necessary measures to fully comply" with the ECJ's July injunction.
While Poland said it would shutter the controversial chamber in August, the defiant October ruling from Poland's Constitutional Tribunal sent shockwaves through the bloc because it challenged the primacy of EU law in the country -- a cornerstone of membership to the union.
The ruling was roundly condemned by the leaders of several EU members.
Polish Prime Minister Mateusz Morawiecki rejected the commission's objections on Wednesday, and notably that Brussels would question the constitutional court’s independence.
Morawiecki said the court "not only fulfills all independence criteria, but it is a Constitutional Tribunal that stands guard of the constitution and ensures that it remains the highest law of the Republic of Poland,” according to Polish news agency PAP.
Polish Justice Minister Zbigniew Ziobro said the step would help transform the EU into a “federal state” in which nation states have less sovereignty, something Poland's ruling party strongly opposes.
“The logic of today’s position of the European Commission is obvious. It is about incapacitating the Polish state and Polish democracy,” Ziobro said.