The European Commission is taking Poland to the European Court of Justice (ECJ) over the country's disciplinary action against judges.
Brussels claims the move is to protect the country's judiciary from political control.
It says the Polish law allows ordinary court judges to be subjected to disciplinary investigations, procedures, and sanctions on the basis of their judicial decisions.
The move today to refer the case to the EU's top court comes after Brussels first launched proceedings in April.
There are parliamentary elections in Poland this weekend in which the ruling conservative Law and Justice (PiS) party — which introduced the controversial reforms to the judicial system — is widely expected to win a second term in office.
"This is a political action and constitutes an attempt to interfere in Polish elections, said Zbigniew Ziobro, Poland's justice minister.
The ECJ ruled in the EU's favour in June in a case over Poland's lowering of the retirement age of Supreme Court judges which it said was "contrary to EU law".
The EU has triggered Article 7 procedures — dubbed the "nuclear" option — against Poland and Hungary and threatened legal action against Romania over concerns about backsliding on judicial independence.
The European Parliament and the European Commission have also backed tying access to EU funds to rule of law as a way of strengthening judicial independence in the bloc.