BRUSSELS (Reuters) - The top European Union court ruled on Wednesday that Poland violated a key democratic tenet of judicial independence in forcing the transfer of a judge critical of the government, as well as denying him the right to appeal to an independent court.

       The ruling by the Luxembourg-based Court of Justice of the EU is the latest blow to the sweeping overhaul of courts carried out by Poland's populist governing Law and Justice (PiS) party.

       PiS says the changes are needed to improve the efficiency of the justice system and rid it of lingering communist influence in the largest eastern EU member states.

       But other EU countries are joined by the United States, international rights watchdogs and human rights advocates in denouncing the changes as a power grab meant to silence critical voices, subdue courts and judges to more government control and undermine democratic checks and balances in Poland.

       Wednesday's ruling concerned Waldemar Zurek, a judge who was transferred to another job without his consent in 2018. He challenged the decision and sought the removal of the judge who dismissed his appeal.

       Zurek said the judge had been nominated as part of a judicial overhaul that violated the democratic separation of powers and failed to ensure judges' independence or impartiality.

       The judge deciding on Zurek's appeal dismissed it alone and without reviewing the case file, the EU court said.

       "The order by which a court, ruling at last instance and sitting as a single judge, dismissed the action of a judge transferred against his will, must be declared null and void if the appointment of that single judge took place in clear breach of fundamental rules," it said in a statement on its ruling.

       (Reporting by Gabriela Baczynska and Anna Wlodarczak-Semczuk, Editing by Catherine Evans and Timothy Heritage)