The Commission -- the EU's executive arm -- said in a statement that it has sent a "letter of formal notice" requesting the Polish government to respond within a month.
The "infringement procedure" was launched on Saturday following the publication of a new Polish law on the organization of ordinary courts on Friday.
The commission identified several provisions in the legislation that might violate European Union law, including “discrimination on the basis of gender” as well as the “discretionary power” of the minister of justice.
The law introduces "a different retirement age for female judges (60 years) and male judges (65 years)," which is contrary to EU law, the commission said.
The statement added that "the new rules allow the minister of justice to exert influence on individual ordinary judges through, in particular, the vague criteria for the prolongation of their mandates thereby undermining the principle of irremovability of judges".
Krzysztof Szczerski, the head of the Polish president's office, told the PAP news agency on Saturday that "with regard to the Polish reform on the judiciary, the EC is looking for pretexts to demonstrate its competence in cases where it obviously does not have it".
"The way the justice system is organized is an internal matter of every state and that is why it differs so much across the EU," Szczerski said.
According to Szczerski, by launching the procedure the commission entered "a road that leads to nowhere."
European Commission Vice-President Frans Timmermans sent a letter on Friday inviting the Polish foreign minister and justice minister for a meeting in Brussels "at their earliest convenience in order to relaunch dialogue."
On Wednesday the European Commission said it would launch infringement proceedings against Poland for alleged breaches of EU law as soon as the law on ordinary courts was published.
The decision followed 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.
Source: PAP/European Commission