BRUSSELS (Reuters) - The European Commission opened a fresh legal case against Poland on Monday over changes to the Supreme Court that it fears will further undermine judicial independence in the largest ex-communist member of the bloc.

FILE PHOTO: Poland's Prime Minister Mateusz Morawiecki is welcomed by European Commission President Jean-Claude Juncker in Brussels, Belgium, March 8, 2018. REUTERS/Yves Herman/File Photo

The European Union’s executive arm gave Poland a month to respond, which means it will not stop changes entering into force on Tuesday under which one-third of Supreme Court judges will retire unless granted an extension by President Andrzej Duda, an ally of the ruling Law and Justice (PiS) party.

“The Commission is of the opinion that these measures undermine the principle of judicial independence, including the irremovability of judges,” it said in a statement.

The Commission has challenged several changes to Poland’s judiciary since the nationalist PiS won power in late 2015, saying they weaken democratic checks and balances. The government says they are needed to reform a system which dates back to communist times.

“This law is currently binding and we will not be changing anything here ... For the time being our stance is that we are right,” Foreign Minister Jacek Czaputowicz said on public television on Monday.

He added that Poland would argue in the European Court of Justice that changes to the judiciary are the responsibility of national governments.

“We are waiting for the decision. Until today Poland has always observed the court’s ruling, including the one on logging in the Bialowieza forest,” Czaputowicz said, referring to another dispute that has soured relations between Brussels and Warsaw.

In April the European Union’s highest court said that Poland broke environmental laws with large-scale logging in the ancient forest.

The EU has opened an investigation into the rule of law in Poland which could, in theory, lead to sanctions. But that is unlikely to happen as any such actions would be vetoed by Poland’s eurosceptic ally, Hungary.

But the latest legal move highlights Poland’s growing isolation in the EU under the PiS and weakens its hand as the bloc is negotiating its next seven-year budget from 2021.