A ruling by Poland's highest court that European Union rules are subordinate to Polish law is an "attack" against the 27-member bloc, France's European Affairs Minister Clement Beaune said on Friday.
"It's very serious, it's not a technical or a judicial issue, it's eminently political," Beaune said during an interview with CNN affiliate BFM TV on Friday.
Poland's Constitutional Tribunal 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.
"When you join a club, you sign a contract, which is called a treaty, which was for that matter approved by a referendum in Poland, it's the Polish people who voted for it," Beaune said. He added that he hoped Poland would not leave the EU, but insisted: "When you are in the club, you respect the rules."
Luxembourg's Foreign Minister Jean Asselborn warned that the Polish government was "playing with fire."
"The development in Poland is very, very concerning... We have to state clearly that this government in Poland is playing with fire," Asselborn said as he arrived for an EU meeting in Luxembourg.
"The primacy of European law is essential for the integration of Europe and living together in Europe. If this principle is broken, Europe as we know it, as it has been built with the Rome treaties, will cease to exist."
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.
"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.
Polish Prime Minister Mateusz Morawiecki welcomed the ruling, saying his country wanted to create a fairer EU.
"[Thursday's] judgment of the Constitutional Tribunal confirmed what literally results from the content of the Constitution of the Republic of Poland. Namely, that constitutional law is superior to other sources of law," Morawiecki said in a statement published on his official Facebook page.
"We have the same rights as other countries. We want these rights to be respected. We are not an uninvited guest in the European Union. That is why we do not agree to be treated as a second-class country," Morawiecki added.
"This is the kind of Union we want and that's the kind of Union we will create," Morawiecki said in the post published in the early hours of Friday.
He also said that Poland wants to stay in the "European family of nations."
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.
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.
Critics say that by challenging the supremacy of EU law, the PiS government not only jeopardizes Poland's long-term future in the bloc but also the stability of the EU itself.
The Polish Constitutional Tribunal took on the case after the prime minister asked it whether EU institutions could stop Poland from reorganizing its judiciary.
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).
PiS party leader Jaroslaw Kaczynski also welcomed the judgment.
"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.
President of the European People's Party and former Polish Prime Minister Donald urged his compatriots to take to the streets in Warsaw to protest the ruling.
"I call on all those who want to defend European Poland to come on Sunday at the Royal Castle square in Warsaw, 6pm. Only together can stop them," Tusk wrote.
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.