LUXEMBOURG - The European Union's top court on Friday ordered Poland to "immediately suspend" its decision to lower the retirement age of its Supreme Court judges, which it said threatens judicial independence.

The decision is the latest salvo in a bitter battle over sweeping judicial changes introduced by the right-wing Law and Justice (PiS) government since it took office in 2015.

They have led the EU to trigger unprecedented proceedings against Poland over "systemic threats" to the rule of law that could see its EU voting rights suspended.

The European Commission, the bloc's executive arm, also took Poland's government to the European Court of Justice (ECJ) for lowering the age at which Supreme Court judges must retire from 70 to 65.

"Poland must immediately suspend the application of the provisions of national legislation relating to the lowering of the retirement age for Supreme Court judges," the European Court of Justice said on Friday.

Warsaw has defended the April 3 retirement law as part of reforms needed to tackle corruption and overhaul a judicial system still haunted by the communist era.

Speaking in Brussels, Poland's right-wing Prime Minister Mateusz Morawiecki said his government had during "the last few hours received the court of justice decision".

"We will certainly respond to it," he said.

"We will see what these (EU) institutions are proposing. When we take them into consideration, several possibilities will be analysed."

The ECJ said the order to suspend retirements "is to apply with retroactive effect" after noting that a number of judges had already been forced to retire.

The order handed down by the court's vice president, judge Rosario Silva de Lapuerta, also freezes Warsaw's appointment of any new judges to replace them.

The top court said its ruling was an interim measure in response to the commission's argument for urgent action in the face of accelerated retirements.

The Luxembourg tribunal, which could impose fines if it finds Poland in breach of EU law, said it will issue a final ruling at a later date.

If the commission's case is upheld, the court said, the Polish Supreme Court's decisions would have been given "without the guarantees connected with the fundamental right of all individuals to an independent court."

The Commission has expressed concern the new retirement age will hasten departure of judges appointed under previous governments, allowing the appointment of figures seen as loyal by Warsaw's current leadership.

The commission has charged Polish judicial reforms undermine "the EU legal order", including member states' mutual recognition of court decisions.

Already in July, the ECJ authorised EU countries to refuse arrest warrants from Poland if they doubt defendants will get a fair trial there.

The law more broadly violates Poland's obligations under the EU treaty, which it signed onto when it joined the bloc in 2004, the commission said.

The new retirement age requires more than a third of current Supreme Court judges to step down, including chief justice Malgorzata Gersdorf.

Calling the law a "purge", Gersdorf has refused to step down, citing a constitutional guarantee that she serve a six-year term until 2020.

"We'll see how the government and the party (PiS) will respond to it," Gersdorf told reporters in Warsaw on Friday referring to the ECJ decision.

She said the government's refusal to drop its controversial reforms was "undermining the dignity of our country."

Warsaw University political scientist Anna Materska-Sosnowska told AFP on Friday that it was "very likely that the PiS will have to change or soften its rhetoric (regarding judicial reforms) to avoid a dip in the polls.

"Poles are staunchly pro-EU and this European Court decision is bound to tarnish the image of the PiS, especially with regard to the elections to the European Parliament" due in May, she said.

The EU first sounded the alarm over Polish judicial reforms shortly after Law and Justice Party, PiS, won elections in 2015.

Brussels has since engaged in more than two years of talks, but Warsaw has largely ignored its warnings that the changes would affect democratic checks and balances.