The British government’s plan for exiting the European Union was thrown into uncertainty on Thursday after the High Court delivered a defeat to Prime Minister Theresa May by ruling that she must seek parliamentary approval before starting the process to leave the bloc.

The court’s decision seemed likely to slow — but not halt — the process for a British withdrawal from the 28-nation union, a decision approved by nearly 52 percent of voters in a June 23 referendum.

Mrs. May had insisted that the government could invoke Article 50 of the Lisbon Treaty, the mechanism for leaving the European Union, without a vote by Parliament. She immediately vowed to appeal the ruling to the Supreme Court of the United Kingdom, which is to hear the appeal in December.