Polish Prime Minister Beata Szydło and Commission Vice President Frans Timmermans shake hands | Janek Skarzynski/AFP via Getty Images Brussels to Warsaw: Get ready for ultimate sanction if top judges fired EU raises stakes in battle over rule of law.

The European Commission warned the Polish government Wednesday it risks being stripped of its EU voting rights if it forces Supreme Court judges out of office.

The Commission’s move raised the stakes in a showdown between Brussels and Warsaw over the ruling Law and Justice party’s plans to exert greater control over the national court system, which the EU's executive body says is a threat to the rule of law in Poland. Those plans hit an obstacle this week when President Andrzej Duda said he would veto two planned laws following large street protests. But the government says it still wants to make the changes.

In addition to its warning over the Polish government's plans for the Supreme Court, the Commission said it was ready to launch infringement proceedings against an overhaul of the lower courts as soon as it becomes law. The Commission uses such proceedings when it believes a member country may not be complying with EU law and they can ultimately result in a case before the European Court of Justice.

The Commission also said the Polish government had one month to address concerns about the current package of laws and previous constitutional reforms.

"The Commission is determined to defend the rule of law in all our member states as a fundamental principle on which our European Union is built. An independent judiciary is an essential precondition for membership in our Union," Commission President Jean-Claude Juncker said.

"If the Polish government goes ahead with undermining the independence of the judiciary and the rule of law in Poland, we will have no other choice than to trigger Article 7," he said in a statement.

Triggering Article 7, the EU’s so-called nuclear option, would be an unprecedented step. In the first instance, the Commission would call upon the European Council to conclude there was a “clear risk of a serious breach” of the EU’s core values.

That would also set in motion a procedure that can lead to a member country losing its right to vote in the Council. However, to get that far would require the approval of all other member countries — and Hungarian Prime Minister Viktor Orbán, an ally of Poland’s ruling party, has said he would block such a step.

Warsaw swiftly hit back at the Commission's warning, branding the criticism unfair.

"We will not agree to any blackmail by EU officials, especially blackmail which is not based on facts," said Rafał Bochenek, a Polish government spokesman. "All the bills prepared by the Polish parliament are in line with the Polish constitution and democratic principles."

Supreme standoff

Under one of the proposed laws that Duda vetoed, all judges on the Supreme Court would be retired, except those designated by Justice Minister Zbigniew Ziobro. The bill also proposed lowering the requirements for future judges on the court — a step critics say would allow the ruling party to pack the court with its allies.

The Supreme Court is Poland’s top court for civil, criminal and military cases; it also confirms election results.

Juncker said the EU could not "accept a system which allows dismissing judges at will."

On the law on the local courts, signed by Duda this week, Commission First Vice President Frans Timmermans said infringement proceedings would be launched as soon as the measure comes into force. Brussels will object on the basis that the law introduces different retirement ages for male and female judges, as this breaks EU law on gender equality.

Relations between Poland and the EU deteriorated in late 2015, after the Law and Justice party won power and then refused to accept five judges chosen for the top constitutional court by the previous government.

That led the Commission to launch an unprecedented three-step probe into the PiS’s move on the Constitutional Tribunal. This probe included the adoption of two Commission recommendations in July 2016 and December 2016.

The recommendation adopted Wednesday “complements” the two previous recommendations, the Commission statement said, and “concerns the lack of an independent and legitimate constitutional review in Poland.”

“As it stands, the Polish authorities have still not addressed the concerns identified in the first two Recommendations,” the statement said.

“Moreover the Polish authorities have now taken additional steps which aggravate concerns about judicial independence and significantly increase the systemic threat to the rule of law in Poland.”

Michał Broniatowski in Warsaw contributed reporting.