Speaking on Wednesday, Commission Vice-President Frans Timmermans said the EU "will not drop this matter" until the Polish government addresses questions about the independence of its judiciary and other issues perceived as backsliding on fundamental political rights.
The Dutch official told a Brussels news conference, "I think this is a substantial challenge for the rule of law in Poland, and the rule of law is the basis upon which a whole European structure is built."
The Commission had planned to debate Poland's rule of law in January, but the discussion was moved forward after an escalation in the standoff between the ruling right-wing Law and Justice party (PiS) and opposition forces.
At the weekend, police in the Polish capital forcefully removed demonstrators, who were blockading the entrance to Parliament in protest against government plans to curb media freedom and weaken Poland's highest court.
The staunchly conservative PiS party has been on a collision course with the EU since it introduced sweeping changes to Poland's institutions.
Timmermans said the Commission considers that whereas some of the issues raised in its last recommendation have been addressed, "important issues remain unresolved" and "new concerns" have arisen in the meantime.
Timmermans said, "The Commission has decided to send additional recommendations to the Polish government because new problems have arisen relating to the rule of law in Poland. The independence of the judiciary is of paramount importance to the rule of law.
"The Commission will not drop this matter and will continue to look for solutions in dialogue with Polish authorities without prejudice to other steps we might have to take if this dialogue continues to remain without results."
He said, "The Commission invites the Polish government to solve the problems identified as a matter of urgency within two months and to inform the Commission of the steps taken to that effect."
The Commissioner, speaking after the last college meeting before Christmas, added, "We feel a strong, strong feeling of solidarity with the Polish people who deserve, like all Europeans, to have an independent judiciary, to have a full separation of powers in their country."
The new EU warnings come as a bitter political standoff in Poland hardens, with opposition politicians vowing to continue a sit-in inside Parliament through the upcoming Christmas and New Year holidays.