Few observers believe that Parliament will go so far as to prevent a departure from the bloc. Lawmakers themselves voted overwhelmingly to hold the referendum and pledged to abide by the results.

The more likely impact could be to weaken Mrs. May’s hold on the negotiating process. Her main priority has been controlling immigration and Britain’s borders, even if that hurts the economy by forcing her nation to leave the European Union’s single market — a “hard Brexit.”

The court’s decision may ultimately force her to compromise, a prospect that led the pound to rise Thursday morning, lifting it from the multidecade lows it had been plumbing in recent weeks.

But it was not immediately clear how the politics would play out. The Conservative government is already split over what kind of future relationship it wants with the European Union, and in general, members of Parliament were not in favor of leaving the bloc in the first place. The government hoped to get the talks started without a major parliamentary debate and potential interference, especially in the House of Lords, where the Conservatives do not have a clear majority.

• What did the High Court decide?

  Prime Minister Theresa May must seek Parliament’s approval before Britain can start the process for leaving the European Union.

• Will ‘Brexit’ still happen?

  The court’s decision seemed likely to slow, not halt, a British exit, although nothing was certain.

• What happens next?

  Mrs. May has vowed to appeal the ruling to the Supreme Court.

• What if she exhausts her appeals?

  Parliament will ultimately have to vote on whether Britain leaves. Though most lawmakers opposed « Brexit, » it would be politically toxic to overturn it now. In any case, Mrs. May’s negotiating position with the European Union could be weakened.

If Mrs. May should find parliamentary opposition intolerable, she might ultimately be tempted to seek an early general election to gain a wider mandate for leaving the bloc, some analysts said. Currently, her Conservative Party holds a slim majority, with 329 seats in the 650-seat Parliament, and many of those members opposed a withdrawal.

The ruling unsettled the proponents of exiting the European Union, who warned against backsliding. Nigel Farage, who resigned as leader of the nationalist U.K. Independence Party after the referendum, said he feared that Britain was heading for a “half Brexit,” and he said he would return to politics in 2019 if the country had not left the bloc by then.

“I see M.P.s from all parties saying, ‘Oh well, actually we should stay part of the single market; we should continue with our daily financial contributions,’ ” he said in an interview on BBC Radio. “I think we could be at the beginning, with this ruling, of a process where there is a deliberate, willful attempt by our political class to betray 17.4 million voters.”

On Thursday, the government said that an expedited appeal would be heard in December by the Supreme Court, Britain’s highest appellate body, and that it was sticking to its timetable for leaving the bloc for now. Yet in the growing environment of constitutional, legal and political uncertainty, the government’s strategy could easily be disrupted.

The ruling was “a severe setback for Theresa May’s government,” said Mujtaba Rahman, managing director for Europe at the Eurasia Group, a political consulting firm. But he added that the government’s timetable could still be met if the Supreme Court ruled in its favor.

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Although Parliament approved holding the referendum, Mrs. May’s critics argued in court that failing to give lawmakers a voice would turn them into bystanders as Britain negotiated its disengagement from the bloc. They also pointed out that, technically, the referendum is not legally binding.

The case, brought by several plaintiffs, including Gina Miller, an investment fund manager, and Deir Dos Santos, a hairdresser, is a constitutional one, about the powers vested in the government, the crown and Parliament. The case is not about whether Britain will or will not leave the European Union, but about the procedure for invoking Article 50, the legal mechanism for leaving the bloc, which provides a two-year period for negotiations.

The plaintiffs argued successfully that leaving the European Union involved the revocation of certain rights granted to Britons by Parliament, and that lawmakers must have a say and a vote before Article 50 is invoked.

In his ruling, the lord chief justice, John Thomas, said, “The most fundamental rule of the U.K. Constitution is that Parliament is sovereign and can make or unmake any law it chooses.”

Oddly enough, this was precisely the case made by those who wanted to end membership, who argued that only by leaving the European Union could Parliament’s sovereignty be completely restored. Now that same argument could delay the very exit so desired by those politicians and their supporters.

The government argued that under residual powers of royal prerogative, which cover international treaty-making, it had the power to invoke Article 50 without a vote in Parliament.

But the court found that invoking Article 50 would essentially repeal the European Communities Act, a 1972 law that allowed for the incorporation of European law into the British legal system, and that only Parliament had the power to do so.

Tim Farron, leader of the Liberal Democrats, welcomed the ruling, adding that it was “disappointing that this government was so intent on undermining parliamentary sovereignty and democratic process that they forced this decision to be made in the court.”

In a statement, he added, “Given the strict two-year timetable of exiting the E.U. once Article 50 is triggered, it is critical that the government now lay out their negotiating to Parliament, before such a vote is held.”

Cian Murphy, a senior lecturer in public international law at the University of Bristol, wrote on Twitter: “The Article 50 judgment from the High Court: a political bombshell but really rather predicable as a matter of constitutional law.”

Although Mrs. May has said that lawmakers will eventually be consulted, many fear it will take place too late to influence the shape of Britain’s new relationship with the European Union.