BRUSSELS — The European Union opened another rule-of-law procedure Monday against Poland over what it sees as flaws in the country's Supreme Court law, intensifying a standoff that could threaten Poland's EU voting rights and funding.

The move comes a day before legislation takes effect that will force the early retirement of 27 of 72 justices of the Supreme Court, or more than a third of them.

The law is the culmination of the ruling populist Law and Justice party's efforts to put Poland's entire court system under its control, a plan it began nearly three years ago. Party leaders claim they are reforming an inefficient and corrupt court system in the grip of an unaccountable caste of judges and insist their changes are in line with European standards.

Critics see the law on Poland's Supreme Court as the most dramatic step in the party's takeover of the courts, giving the ruling party the power to stack them with loyalists. One of the court's jobs is to verify election results, and critics say the new law marks a serious reversal for democracy.

In announcing its procedure, the European Commission, which polices EU law, said the measures "undermine the principle of judiciary independence."

Since "there was no step from the Polish side to reverse them, we made the decision to launch the infringement procedure as a matter of urgency to defend the independence of the Polish judiciary," EU spokesman Margaritis Schinas said.

Poland now has a month to respond, but if it does not reverse course, a future step would involve the Commission suing Poland at the EU Court of Justice. If Poland were to lose, it could face heavy fines.

Poland has insisted that how it organizes its judiciary is an internal matter that the EU has no right to interfere in. But EU officials have sharply disagreed, saying Poland willingly signed on to EU rules when it joined the community, and that the courts must be counted on to also uphold EU contracts and law.

Mass protests erupted in Poland last summer over new judicial laws, with many of the same Poles who opposed communism three decades ago taking to the streets. The domestic upheaval as well as concerns by the European Commission prompted authorities to concede to some changes, though the main thrust of the legislation has remained the same.

Government spokeswoman Joanna Kopcinska said Monday that the government of Prime Minister Mateusz Morawiecki is fulfilling its promise to voters to clean up a broken justice system — changes she said previous governments should have made but didn't.

"Maybe we would not have had this discussion (with the EU) had they been done earlier," Kopcinska said.

On the eve of the changes, Supreme Court spokesman, judge Michal Laskowski, said it was not yet clear if the president would use his power to immediately force the retirement of the judges, including the court president Malgorzata Gersdorf. They have vowed to show up to work as usual.

"We can expect all sorts of reactions, from nothing happening in the coming days, to a lot happening, including the use of force," Laskowski said.

The European Commission has also launched a separate procedure against Poland known as Article 7, which opens the way for possible sanctions and a suspension of voting rights. Hungary, however, has vowed to use its veto to prevent that step against Poland.

With few effective tools left, the EU earlier this year vowed to tie future subsidies to the rule of law, something that could cause Poland to lose some of its future funding — money that has helped fuel 14 years of fast economic development.