A contested reform in Poland concerning disciplinary cases involving judges goes against European Union law, the advocate general of the European Court of Justice said on Thursday.

Advocate General Evgeni Tanchev said the legislation did not guarantee the "independence and impartiality" of a newly-established "disciplinary chamber" of the Supreme Court.

The Polish law on reforming the judiciary, which came into force in February last year, prevents judges from referring questions of law to the European Court of Justice and creates a body that rules on judges' independence without regard to EU law.

It also set up a "disciplinary chamber" to oversee Polish supreme court judges, with the power to lift their immunity to expose them to criminal proceedings or cut their salaries.

Poland's right-wing populist government implemented the reform despite opposition from Brussels, saying it is a way of dealing with the vestiges of communism in the judicial system.

Critics say it undermines the independence of the judiciary and is intended to intimidate judges who do not toe the line.

Brussels has for years tried to bring Poland back into line with what it considers democratic norms and the reform of the judiciary is a particularly bitter point of contention.

Poland's Deputy Justice Minister Sebastian Kaleta reacted angrily to the advocate general's opinion statement, calling it "political theatre" and an "attack on Poland".

"We consistently see the elites in Brussels trying to interfere in Poland's sovereignty over the judiciary," he said.

Kaleta also said Tanchev was "worth looking into" and had enjoyed "a stellar career as a lawyer in communist times".

Kaleta said he was looking forward to a landmark ruling from Poland's Constitutional Court on the issue expected next week, which could challenge the primacy of EU law in Poland.

"We will definitely defend Polish sovereignty," he said.

A full ruling from the European Court of Justice is expected in the coming weeks.

Opinions from the advocate general are considered by the court but it is not bound to follow them.