Poland's highest court ruled on Thursday that some parts of EU treaties are incompatible with the Polish constitution, challenging a pillar of European integration and sharply escalating a dispute between Brussels and Warsaw.

The European Commission said the ruling raised serious concerns about the primacy of EU law, setting it on a full collision course with Poland's nationalist rulers after years of legal and political wrangling.

Poland's Law and Justice (PiS) party government is embroiled in a battle of values with Brussels, focused on disputes over the independence of courts, media freedoms, LGBT rights and other issues.

Critics say that by going further and challenging the supremacy of EU law, the PiS government not only jeopardises Poland's long-term future in the 27-nation bloc but also the stability of the EU itself.

PiS denies this and says it has no plans for a "Polexit".

Poland's Constitutional Tribunal took on the case after Prime Minister Mateusz Morawiecki asked it whether EU institutions could stop Poland from reorganising its judiciary.

On Thursday, Judge Bartlomiej Sochanski said: "The EU Treaty is subordinate to the constitution in the Polish legal system ... and, like any part of the Polish legal system, it must comply with the constitution."

PiS party leader Jaroslaw Kaczynski welcomed the ruling.

"In Poland the highest legal act is the constitution and all European regulations that are in force in Poland ... must comply with the constitution.

"This also applies to the judiciary and the European Union has nothing to say here," he told reporters.

POWERS, SAFEGUARDS

Brussels accuses the PiS government of undermining judicial independence during sweeping reforms. The party says the changes are meant to make the courts more efficient and rid them of the last vestiges of Communist-era influence.

In its ruling, the Constitutional Tribunal said it has a right not only to check the constitutionality of EU law but also the rulings of the Court of Justice of the European Union (CJEU).

The European Commission said it would analyse the ruling before deciding on its next steps, without going into more detail.

"The Commission will not hesitate to make use of its powers under the Treaties to safeguard the uniform application and integrity of Union law," it added.

The Tribunal started hearing the case in July but had adjourned it four times before Thursday's sitting.

Some critics say the delays may have been aimed at putting pressure on Brussels to accept Warsaw's National Recovery Plan, an aid programme aimed at helping EU economies shake off the impact of the pandemic but whose funding is linked to compliance with EU rule of law and democracy standards.

The European Commission has already approved the majority of national plans but has withheld approval for Poland and Hungary over concerns about those standards.

Earlier this month EU officials said that the European Commission may approve national recovery plans of Poland and Hungary in November, but will set conditions linked to respecting the rule of law.