The move is part of escalating EU pressure on Poland over what Brussels sees as a threat not just to Warsaw's democratic standards but to those across the 28-nation bloc.
"The European Commission launched an infringement procedure against Poland by sending a letter of formal notice," the EU executive said.
Poland published the law reorganising its ordinary courts on Friday.
The EU statement said Poland's governing Law and Justice Party (PiS) had one month to respond to the Commission letter which "raises concerns that ... the independence of Polish courts will be undermined."
The commission pointed to the justice minister getting "discretionary power to prolong the mandate of judges who have reached retirement age as well as to dismiss and appoint court presidents."
Other concerns, it said, include "discrimination on the basis of gender" by setting the retirement age at 60 for female judges and at 65 for their male counterparts.
The justice minister, it said, can "retain influence" over them by extending at any time their mandates -- for up to 10 years for women and up to five years for men.
The action taken by the Commission could lead to Poland being hauled before the bloc's highest court, the European Court of Justice, and eventually being given a fine.
The Commission is also prepared to take more severe action if Warsaw forges ahead with deeper court reforms.
EU move 'unfounded'
Poland's deputy foreign minister for European affairs, Konrad Szymanski, told the PAP news agency that the EU decision was "unfounded," adding the new common courts law carried proper guarantees.
The action in Brussels was expected after Polish President Andrzej Duda on Tuesday signed into law a measure allowing the justice minister to unilaterally replace the chief justices of the common courts.
However, Duda surprised many inside and outside the PiS government when he vetoed a bill that would have reinforced political control over the Supreme Court and another allowing parliament to choose members of a body designed to protect the independence of the courts.
Polish Prime Minister Beata Szydlo has vowed to push ahead with all the reforms despite the vetoes.
European Commission First Vice President Frans Timmermans on Wednesday warned the "commission is ready to immediately trigger the article 7 procedure" if Supreme Court justices are sacked.
Article 7 is a never-before-used EU process designed to uphold the rule of law, a so-called "nuclear option" that can freeze a country's right to vote in meetings of EU ministers.
The chances are slim that its voting rights could actually be suspended. Populist Hungarian Prime Minister Viktor Orban has vowed he would instantly veto any such move by the EU.
The escalating standoff with Poland threatens to deepen an east-west split in the EU.
Hungary itself faces EU legal action over laws targeting education and foreign civil society groups, while Poland, Hungary and the Czech Republic also face action for ignoring the bloc's migrant relocation quotas.
Timmermans also sent a letter on Friday to Poland's Foreign Minister Witold Waszczykowski and Justice Minister Zbigniew Ziobro inviting them to Brussels to relaunch a long drawn-out dialogue on the judiciary reforms.
"The Commission's hand is still extended to the Polish authorities, in the hope of a constructive dialogue," Timmermans said, according to the commission.
The legal reforms have triggered mass street protests in Poland and raised fears for the rule of law in one of the EU's leading eastern former communist states.
Brussels and Warsaw have been at loggerheads over the legal changes ever since the right-wing PiS took power in 2015 and announced reforms to Poland's constitutional court.
EU Justice Commissioner Vera Jourova has expressed fears the "whole EU system of mutual recognition of court decisions" could be undermined if Polish judicial independence were undermined.
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