Israel’s supreme court has ruled that the prime minister, Benjamin Netanyahu, must fire a key ally from the country’s new cabinet, presenting the Israeli leader with a potential coalition crisis and deepening a rift over the power of the courts. Ten of 11 judges on the high court found that Aryeh Deri, the influential head of the ultra-Orthodox Shas party who has served repeatedly in Netanyahu’s previous governments, is disqualified from serving as a minister after he was convicted last year for tax offences and placed on probation as part of a plea deal. Deri has pledged not to quit and met Netanyahu after the ruling. “Most of the judges on the panel decided that this appointment suffers from extreme unreasonability, and therefore the prime minister must remove Deri from his position,” the court said in a statement. Deri was defiant. “When they close the door on us, we’ll get in through the window. When they close the window, we’ll break through the ceiling,” he said. The ruling came as a dispute continued over proposed sweeping changes to the country’s legal system. One such proposal under consideration is the elimination of the court’s “reasonability” test when reviewing government decisions. Critics say the various changes at issue would place too much power in the hands of the government and weaken the supreme court. Proponents said they would correct a power imbalance between the executive and judicial branches. Netanyahu will now have to decide whether he abides by the court ruling and fires his key ally Deri – or takes the dispute with the judicial system up a notch and defies it. A spokesperson for Netanyahu had no immediate comment. But the leaders of the parties in the ruling coalition said the ruling was disrespectful to voters after the 1 November election. In a statement, they vowed to act “in any legal way that is available to us and without delay to correct the injustice and the severe damage caused to the democratic choice and the sovereignty of the people”. Dr Amir Fuchs, senior researcher at the Israel Democracy Institute, a Jerusalem thinktank, said Netanyahu is unlikely to ignore the ruling because then he’d be in contempt of court and there is no appealing against a supreme court decision. “I am sure that he will abide by the ruling. It doesn’t mean that he will respect the ruling,” Fuchs said. “What will probably happen is that they will do very quick legislation that will enable him to appoint Deri again.” Critics said such a move would bend the rules to accommodate someone with a conviction and could encourage corruption among politicians. The ruling carries potentially troublesome consequences for Netanyahu’s coalition. Yakov Margi, a Shas cabinet minister, told Kan public radio: “If Aryeh Deri isn’t in the government, there isn’t a government.” In a move that was seen as crucial to bringing the governing coalition together, Israeli legislators last month changed a law that prohibited someone on probation from being a cabinet minister. That cleared the way for Deri to join the government but prompted the supreme court challenge.