BRUSSELS/WARSAW, Oct 6 (Reuters) - A European court found on Wednesday that Poland broke democratic norms by appointing judges who are not sufficiently independent, and declared a decision by one such judge to be void, escalating a conflict between the EU and Poland"s ruling nationalists. Warsaw and its critics alike said the ruling, ostensibly in the narrow case of a judge who fought his transfer to another job, could have wide implications for Poland"s legal system. The Luxembourg-based Court of Justice of the EU (ECJ) found that Poland had denied judge Waldemar Zurek, who served in a civil court in Krakow, his right to a fair appeal of his transfer. A Supreme Court judge who reviewed Zurek"s case had himself been appointed in a way that "cast reasonable doubts... as to his neutrality", which meant Zurek"s dismissal "must be declared null and void", it ruled. Polish Prime Minister Mateusz Morawiecki described the ruling as "an attempt to hit at the very heart of the social and legal system", and said it could affect "hundreds of thousands" of Polish legal decisions. "Naturally, we cannot allow that," he told a news conference in Slovenia, without explaining what steps Poland would take. Poland"s Constitutional Tribunal is separately hearing a challenge brought by Morawiecki"s government against the principle that EU law supersedes Polish law, a pillar of the European system. "GAME OVER" Wednesday"s ECJ ruling leaves the next steps in the hands of Polish courts. But the EU"s executive commission can take legal action to enforce it, if it determines that Poland has ignored it. Poland and Hungary have been under investigation by the EU over their record on the rule of law, with Brussels threatening to withhold European funds if the two eastern countries" nationalist administrations do not comply with EU standards. Poland"s governing Law and Justice (PiS) party has introduced sweeping changes giving politicians more power to pick judges, saying the overhaul is needed to drive out holdovers from the communist era that ended three decades ago. The opposition and other EU countries denounce the changes as a power grab to silence critics, subject courts to more government control and undermine democratic checks and balances. They also accuse Poland of failing to meet European standards with laws restricting the rights of women and gays. Warsaw says those laws reflect the predominantly Catholic country"s conservative culture. The ECJ ruled last July that a new system for disciplining judges was being used to pressure judges or to exert political control over their decisions and hence undercut EU laws on judicial independence. Wednesday"s ruling confirms that new disciplinary bodies for judges "should be considered illegal and their rulings void. Game over," said an opposition lawmaker, Kamila Gasiuk-Pihowicz. The judges" association Iustitia, which has accused PiS of degrading the courts, said more than 1,000 judges had been nominated since Warsaw started staffing panels with judges appointed by government officials rather than by other judges. Zurek said the PiS government had an authoritarian tilt and that he was very pleased with the ECJ ruling. Additional reporting by Pawel Florkiewicz, Writing by Gabriela Baczynska, Editing by Peter Graff and Jon Boyle