The European Commission also said on Wednesday it was ready to trigger a formal warning by the EU if Poland dismisses or forces the retirement of Supreme Court judges.
The move follows over a week of street protests in Poland against the country’s ruling conservatives' planned changes to the judicial system.
Poland's governing Law and Justice (PiS) party has said sweeping changes are needed to reform an inefficient and sometimes corrupt judicial system tainted by the communist past, accusing judges of being an elite, self-serving clique often out of touch with the problems of ordinary citizens.
But opponents have accused Law and Justice of aiming to stack courts with its own candidates and to dismantle the rule of law.
European Commission President Jean-Claude Juncker said on Wednesday: "The Commission is determined to defend the rule of law in all our member states as a fundamental principle on which our European Union is built."
He added: “An independent judiciary is an essential precondition for membership in our Union. The EU can therefore not accept a system which allows dismissing judges at will.”
Poland will not accept EU 'blackmail'
Government spokesman Rafał Bochenek said Poland "does not agree to any blackmail by EU officials, especially blackmail which is not based on facts."
He added: “All the bills prepared by the Polish parliament are in line with the Polish constitution and democratic principles."
Foreign Minister Witold Waszczykowski said the degree of the commission's interference in the Polish legislative process was unacceptable.
Polish President Andrzej Duda on Monday said he would veto two controversial government-backed bills including one that would have forced the Supreme Court's existing justices into retirement while giving the president powers to choose who to reinstate.
However, he signed into law reforms that change the way the heads of district and appeals courts are appointed and dismissed, giving more power to the justice minister.
Duda announced that he would draw up new bills on the Supreme Court and the influential National Council of the Judiciary (KRS), a body tasked with safeguarding the independence of courts and judges.
Series of clashes
The commission's move on Wednesday was the latest in a series of clashes between Brussels and Warsaw. Poland is already embroiled in a row with the EU over sweeping changes to the country’s Constitutional Tribunal.
The European Commission said that it “substantiates its grave concerns on the planned reform of the judiciary in Poland in a Rule of Law Recommendation addressed to the Polish authorities.
“In the Commission's assessment, this reform amplifies the systemic threat to the rule of law in Poland already identified in the rule of law procedure started by the Commission in January 2016. The Commission requests the Polish authorities to address these problems within one month.”