The European Commission launched legal action on Wednesday after Poland's constitutional court appeared to challenge the primacy of EU law.
Brussels has repeatedly warned Prime Minister Mateusz Morawiecki's nationalist government that its legal reforms undermine the rule of law in Europe, but has been rebuffed.
Now, a formal case has been launched that could see Warsaw facing huge fines over the behaviour of its constitutional court.
"EU law must be equally applied across the Union," EU vice-president for values and transparency Vera Jourova said in a social media post announcing the move.
The basis of the case is two rulings by the Polish Constitutional Tribunal in July and October 2021 which judged that some aspects of EU treaty law are incompatible with the Polish constitution.
The European Commission argues these rulings expressly challenge the primacy of EU law.
"The Commission's objective is to ensure that the rights of Polish citizens are protected and that they can enjoy the benefits of the EU in the same way as all EU citizens," the commission said.
"Primacy of EU law ensures equal application of EU law across the Union."
- 'Blackmail' -
Brussels has already had run-ins with the Polish tribunal, which it believes damages the independence of Poland's judiciary and extends the government's influence over the courts.
"The Commission also considers that the Constitutional Tribunal no longer meets the requirements of an independent and impartial tribunal previously established by law," it said.
Poland's hardline justice minister, Zbigniew Ziobro, reacted with fury and accused the EU of conspiring with Germany to undermine Polish independence and create a centralised federation.
"This demand and the blackmail in this case is aimed at eliminating Polish statehood as we know it, within the meaning of the Polish constitution," he said.
But Poland's EU affairs minister, Szymon Szynkowski vel Sek, later sought to tone down the government rhetoric, saying Warsaw was willing to engage in an "assertive" dialogue with the Commission.
"This decision was to be expected, it does not come as a surprise to us," he told reporters, adding that Poland will "present its arguments" before the EU court.
Warsaw has already been ordered to pay daily EU fines of a million euros for failing to apply a European court of Justice ruling demanding changes to disciplinary procedures for judges.
The fines now total 440 million euros (around $470 million), part of which is to be deducted from EU funding support for Poland.
Warsaw has passed a law abolishing the Supreme Court's disciplinary chamber for judges, but this did not go far enough for Brussels to rule the court judgment had been applied.