Poland's president has signed into law an amendment to how its constitutional court makes rulings, a move critics say will erode checks and balances in government powers and paralyse the highest judicial body.

It was the latest development in a constitutional crisis which began when the conservative nationalist Law and Justice (PiS) party, winner of October's parliamentary election, appointed five judges to the 15-member court.

The government said the step was necessary for the court to properly reflect the results of the election.

The appointments were labelled illegal by opposition leaders, triggered major public protests, increased investor jitters and drew accusations from rights activists that the ruling majority was undermining democratic checks and balances.

The amendment, drafted and passed by PiS and now signed into law by President Andrzej Duda, a close ally of the party, requires the 15-member court to adopt most of its rulings by a two-thirds margin with at least 13 judges present.

This could force the head of the constitutional court to include the five judges chosen by the PiS-controlled parliament in the court's composition or leave the court unable to pass rulings.

Hitherto, as few as five judges could be assigned to and vote on any particular case.

PiS lawmakers said the amendment was needed to clear up legal confusion after the constitutional court ruled earlier this month that another amendment passed by the former parliament partially violated the constitution.

The amendment will improve the constitutional court's reputation and rulings by an increased majority will have a stronger legal standing, Mr Duda said.

"It's hard for me to understand the situation we've had until now, in which, de facto (just) three judges could rule on the legal fate of legislation passed by the parliament elected by the people."

The European Union executive has expressed concern over the amendment, calling for its introduction be postponed.

Poland was long seen as one of the strongest supporters of the EU's democratic, rule-of-law principles among former Soviet bloc states now part of the bloc. However, Poland's new ruling conservatives appear bent on strengthening their sway over state institutions and the media to reflect the nation's Catholic values and independence from Brussels.