British Plan to Exit E.U. Is Thrown Into Turmoil by Ruling

The British government’s plan for leaving the European Union was thrown into uncertainty on Thursday after the High Court ruled that Parliament must give its approval before the process can begin.

The court’s decision seemed likely to slow — but not halt — the British withdrawal from the bloc, a step approved by nearly 52 percent of voters in a June referendum.

Nevertheless, the decision on Thursday was a significant blow to Prime Minister Theresa May, who had planned to invoke Article 50, the mechanism for leaving the European Union, no later than the end of March and to prepare for the negotiations mostly behind closed doors.

If the court’s ruling is upheld — the government immediately vowed to appeal — that plan would be thrown into disarray, analysts said, as Mrs. May would be forced to give Parliament a detailed strategy for negotiating the British departure, known as Brexit. She has adamantly resisted doing so, arguing that this would impede her flexibility in the negotiations, preventing Britain from getting the best possible deal.

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