Poland's Constitutional Tribunal could rule on Wednesday on whether the country's constitution or European Union treaties take precedence, a key judgement which an EU commissioner said was holding up the release of EU funds to Warsaw.
Poland is involved in a series of disputes with the EU on issues ranging from courts and media freedom to LGBT rights, as critics accuse its nationalist government of inching toward an exit from the bloc.
Prime Minister Mateusz Morawiecki brought the case as part of a dispute with the EU over changes to Poland's judicial system, which Brussels says may undermine judicial independence.
The primacy of European laws over national ones is a key tenet of European integration.
Warsaw says Brussels has no right to interfere with the judicial systems of EU member states and argues the reforms were needed to remove communist-era influence in the judiciary and speed up proceedings.
Poland's ruling Law and Justice (PiS) party leader Jaroslaw Kaczynski says the EU's insistence on the primacy of EU law over national legislation undermines the foundations of Poland's sovereignty and its constitutional order.
EU FUNDS AT STAKE
While some warn of a potential "Polexit", Poland is unlikely to leave the bloc any time soon. There is no legal way to throw countries out of the EU and most Poles support membership.
A survey this month showed 88% of Poles believed Poland should remain in the EU and only 7% favoured pulling out, but nearly a third believed leaving as a possibility.
But Poland could lose EU funds that have helped its economic development. European Economic Commissioner Paolo Gentiloni has said the court challenge is holding up 57 billion euros ($66 billion) in EU recovery aid to Warsaw.
The EU's top court ruled in July that a Polish disciplinary chamber for judges was illegal, a day after the Constitutional Tribunal in Warsaw ruled that Poland should ignore a previous demand to stop the chamber operating.
Following a threat of financial penalties from the European Commission, Poland said it would disband the chamber, but hasn't provided details.
Warsaw is also waiting to hear from the Commission after five Polish regions refused to back out of declarations they would remain "LGBT-free", which means they may lose EU funds.
On Monday, the EU court ordered Poland to pay a 500,000 euro daily fine for defying a ruling to halt the Turow lignite mine on the border with the Czech Republic. Warsaw vowed to keep the mine running despite the penalty.
(Reporting by Anna Wlodarczak-Semczuk and Joanna Plucinska; Editing by Giles Elgood)
