LONDON — Britain's plans to leave the European Union hit a large speed bump Thursday, as the High Court of Justice ruled that the government can't start exit negotiations without a vote in Parliament.

The judgment deepened Britain's divide over Europe, raising hopes among pro-EU politicians that they can soften the terms of the U.K.'s withdrawal from the bloc. “Leave” campaigners say any attempt to do that would be a betrayal of the voters' decision.

The government immediately said it would ask the Supreme Court to overturn the ruling.