Wednesday's notice from the European Commission, which is the EU's executive, is seen as a step toward sending the case to the European Court of Justice in what would be an aggravation of the dispute.
In April, the commission requested explanations from Poland on the 2017 law that introduced disciplinary measures for judges. In the commission's opinion, they subjected the judiciary to political control, in violation of rule of law principles and EU rules.
The new letter requires Poland to comply with the commission's opinion within two months or face the possibility of being brought before the EU court.
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