The European Commission said on Wednesday it would start legal action against Poland over a law reforming the judiciary that undermines the independence of judges and therefore breaks EU treaty rules.

The Commission also sent Poland a new recommendation under a separate rule-of-law monitoring procedure asking it to address its concerns about several issues questioned by the EU executive since the start of 2016 and gave Warsaw one month to respond.

"An independent judiciary is an essential precondition for membership in our Union," Commission head Jean-Claude Juncker said in a statement. "The EU can therefore not accept a system which allows dismissing judges at will."

"If the Polish government goes ahead with undermining the independence of the judiciary and the rule of law in Poland, we will have no other choice than to trigger Article 7," Juncker said referring to a legal process of suspending Poland's voting rights in the 28-nation EU.