The European Commission says it will open a case against Poland over constitutional court legislation which has been criticized as running counter to fundamental principles of the rule of law.

EU Commissioner Frans Timmermans said his executive "will carry out a preliminary assessment of this matter under the rule of law framework," the first step in a drawn-out procedure that could ultimately lead to suspending Polish voting rights in the EU.

Timmermans stressed though that the EU would be seeking dialogue with Poland first. In December, the ruling Law and Justice party, which has a full parliamentary majority, took steps to gain influence in the constitutional tribunal, which is supposed to be an independent arbiter with the power to block the government's legislation.