The head of Poland's Supreme Court called on judges to return to the nation's top bench after the European Union's top court ordered Poland to immediately reverse a law lowering the retirement age.

All justices who had been forced into retirement by the law were ordered to return to their positions, Supreme Court First President Malgorzata Gersdorf said in a public statement on Monday.

Several of the justices had already appeared in the court on Monday, said court spokesman Michal Laskowski.

The European Commission had referred Poland to the European Union's top court, the European Court of Justice (ECJ), arguing that the new law introduced in April is in breach of fundamental EU values.

Brussels and Warsaw have been at loggerheads since late 2015 due to concerns that reforms introduced by Poland's conservative government place the judiciary under excessive political influence.

The reform in question lowers the retirement age of Supreme Court judges from 70 to 65, a measure that has triggered the departure of more than 23 judges since early July, including Gersdorf.

Poland's ruling Law and Justice party (PiS) says it is purging the court of corrupt judges, but opponents accuse it of trying to remove critical voices. PiS also argues that, under EU rules, the organization of the judiciary is a national matter.

As an interim measure, Poland must "immediately suspend" the law on early retirement, applying the decision "with retroactive effect to the judges of the Supreme Court concerned by those provisions," the ECJ wrote in a statement on Friday.

The Luxembourg-based judges must still rule on whether the Polish reform is in breach of EU rules, a process that could take weeks or months.