EU economy commissioner Paolo Gentiloni said the infringement proceedings targeted Poland for breaching the primacy of EU law and for deciding that certain articles of EU treaties were incompatible with Polish laws.

The step escalates a long-running feud between Warsaw and Brussels over Poland's perceived backsliding on EU democratic norms.

Brussels is already withholding approval of coronavirus recovery funds for Poland over the row.

Poland's Deputy Justice Minister Sebastian Kaleta hit back by calling the EU announcement "an attack on the Polish constitution and our sovereignty".

- Persistent defiance -

Legal action from Brussels was expected given persistent defiance from Poland's Constitutional Court to the European Court of Justice (ECJ).

The ECJ has already ruled against Poland for implementing a mechanism to lift the immunity of judges in the Constitutional Court and to sack any not deemed acceptable by the parliament dominated by the governing populist Law and Justice party.

The European Commission is also upset over a 2019 Polish law that prevents Polish courts applying EU law in certain areas, and from referring legal questions to the ECJ.

Gentiloni told a media conference the Polish moves "breached the general principles of autonomy, primacy, effectiveness and uniform application of Union law and the binding rulings of the Court of Justice".

The European Commission, he said, considers the Polish Constitutional Court "no longer meets the requirements of an independent and impartial tribunal established by law, as required" by a fundamental EU treaty.

He said Poland had two months to respond to a formal letter setting out the grounds of the infringement procedure.

In the event of no satisfactory reply, the matter could be sent to the ECJ.

While there is no option to kick Poland out of the EU for not respecting the bloc's laws, it could be hit with daily fines for non-compliance.

But Poland and Hungary ,another eastern EU member accused of undermining democratic norms , have a pact mutually shielding each other from more extreme EU punishment, such as removing their voting rights in the bloc.

Hungary, too, faces delays to receiving EU coronavirus recovery money because of its own defiance of EU rules.

Both countries have threatened to block EU business in retaliation for Brussels' actions.

Gentiloni said he was "confident" the rows with Warsaw and Budapest would not degenerate into a "tit for tat" cycle , but cautioned "we can't exclude anything".