The EU executive has given Poland an official warning that changes to its constitutional court endanger the rule of law in the country.
Frans Timmermans, vice-president of the European Commission, said he had written to the Polish government warning that recent alterations to the workings of Poland's highest court posed "a systemic risk to the rule of law".
The publication of a formal opinion ratchets up pressure on Poland and marks the first time that the EU executive has criticised a member state under its rule-of-law procedure.
After Poland’s Law and Justice (Pis) party came to power, the Polish parliament passed a law allowing the government to appoint the judges of its choosing to the highest court and not recognise those chosen by its predecessor, the liberal Civic Platform party.
Legal experts advising the Council of Europe have concluded that the changes breach the rule of law, democracy and human rights.
If Poland refuses to back down, it could face the ultimate sanction of being stripped of EU voting rights, although Brussels is keen to avoid that scenario.
Mr Timmermans said he had spoken to the Polish prime minister, Beata Szydlo, on Tuesday night and was pleased that she wanted “our dialogue to continue”.
Declaring himself to be “dispassionate and legal”, he said the commission was the guardian of the EU treaties and had a duty to ensure the rule of law.
"The European Union is built on common values enshrined in the treaties," he said. "Making sure the rule of law is observed is a collective responsibility of all EU institutions and all member states."
The head of the ultra-conservative Law & Justice party, Jaroslaw Kaczynski, has condemned the EU action as “a made-up” procedure that “can be challenged in the court of justice of the European Union at any moment”. Making his disagreement clear, Mr Timmermans said all member states needed to follow the EU treaties they had ratified.
The EU introduced its rule-of-law procedure in 2014, after years of struggling to deal with lapses in democratic standards among member states.
In 2011, Hungarian prime minister Viktor Orban agreed to change a media law restricting press freedom, after talks with the commission.
Mr Timmermans called on the Polish government to act quickly. Polish leadership was needed in the EU, he said, and should not be distracted by an issue that can be solved.
Guardian service