Legislative changes adopted in Warsaw since April in a bid to solve the row "are not sufficient to eliminate the clear risk of a serious breach of the rule of law," Timmermans told the European Parliament.
Poland's right-wing Law and Justice (PiS) government began making changes to the judiciary after coming to power in late 2015. It says the reforms are needed to combat corruption and overhaul the judicial system still haunted by the communist era.
With the lower courts already under threat, "it is now the Polish Supreme Court which is at risk of coming under political control," warned Timmermans, who leads the Polish dossier in the commission, the EU's executive arm.
Around a third of Poland's supreme court judges risk being fired or forced to retire next month under the reforms, he said. Timmermans added that "last week the Commission therefore asked the Council to organise a formal hearing of Poland" in line with article seven of the EU treaty.
The proceedings could much further down the line lead to the never-before-used "nuclear option", in which Poland's EU voting rights are suspended.
Three former Polish presidents, including anti-communist icon Lech Walesa, earlier on Wednesday urged the EU to defend the rule of law in their country, ahead of the decisions on the Supreme Court.
The row underlines growing east-west tensions within the European Union, with former Soviet bloc states like Poland and Hungary refusing to toe the Brussels line on several thorny issues including judicial and media independence as well as immigration.
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