A British court has dealt a blow to Prime Minister Theresa May’s E.U. exit plan. The three-judge panel sided with plaintiffs who contended that Parliament must first weigh in. This could lead to a significant delay to Brexit. (Griff Witte,Jason Aldag/The Washington Post)

Britain’s plan for getting out of the European Union was thrown into doubt Thursday as a senior court ruled that Prime Minister Theresa May will need to get Parliament’s approval before she acts.

The surprise decision introduced new uncertainty to a process already fraught with complication and threatened to derail May’s timetable of triggering Article 50, the never-before-used mechanism for exiting the E.U., by the end of March. 

It also boosted the odds that the prime minister, in office only since July, will have to call a fresh election next year to win the mandate she needs to launch E.U. divorce talks. 

The decision drew immediate condemnation from pro-Brexit politicians, who warned of an angry backlash from voters who favor leaving the 28-member bloc and had thought the matter was settled when they opted in a June referendum to get out. 

Pro-E.U. leaders, meanwhile, showered the ruling with praise, and the pound jumped on hopes that Brexit might be postponed — or somehow avoided altogether. 

A statement from May’s office at 10 Downing Street said it was “disappointed” by the ruling and would appeal to the Supreme Court. Justices are expected to take the case next month. 

At the heart of the legal dispute is a clash between direct and representative democracy. Although the British opted for Brexit by a margin of 52 percent to 48 percent, a large majority of members in Parliament wanted Britain to stay in the E.U. By giving Parliament a voice, the London-based High Court handed power back to a group that is skeptical or even hostile toward the very idea of Brexit.

Mujtaba Rahman, Europe director for the Eurasia Group political consultancy, said Thursday that lawmakers will be reluctant to incur voters’ wrath by going directly against their will and blocking exit plans.

Nonetheless, he described the court’s decision as “a severe setback for Theresa May’s government.”

If the Supreme Court upholds the judgment on appeal, Rahman wrote in a Thursday analysis, then pro-E.U. lawmakers could use the process to “seek to tie May’s negotiating hand.”

One option for May, in turn, could be to call a general election next year “to ask the public to endorse her negotiating goals — in effect, to use an election to override Parliament,” he added.

That would be a sharp break from the plan May has repeatedly outlined. She intends to trigger Brexit on her own, without Parliament’s input, and has ruled out an early election.

Thursday’s decision instantly threw that plan into disarray. A three-judge panel representing England and Wales dismissed government lawyers’ arguments that May has the executive power necessary to launch Brexit talks on her own and sided with a group of plaintiffs who contended that Parliament must weigh in first.

“The most fundamental rule of the U.K.’s constitution is that Parliament is sovereign and can make and unmake any law it chooses,” the judges wrote. “As an aspect of the sovereignty of Parliament it has been established for hundreds of years that the Crown — i.e. the Government of the day — cannot by exercise of prerogative powers override legislation enacted by Parliament.”

The court's decision stunned British political and legal observers — just as the referendum outcome also defied predictions that voters would favor staying in the E.U. Until Thursday, most analysts had predicted the court would side with the government. The High Court in Northern Ireland had ruled as recently as last week that May's government could bypass Parliament.

Thursday’s ruling sparked an immediate rally in Britain’s beleaguered currency. The pound has been battered since the referendum and has been one of the worst-performing currencies in the world this year. 

Mark Carney, governor of the Bank of England, warned again Thursday of likely inflation next year and said the ruling is “an example of the uncertainty that will characterize this process.”

Brexit advocates quickly denounced the decision, saying it amounted to a betrayal of the public’s will.

“I now fear every attempt will be made to block or delay triggering Article 50,” tweeted Nigel Farage, a longtime Brexit champion. “They have no idea the level of public anger they will provoke.”

Suzanne Evans, a candidate to succeed Farage as leader of the U.K. Independence Party, added a condemnation of “activist judges” who “attempt to overturn our will.”

“Time we had the right to sack them,” she wrote.

Pro-E.U. politicians, meanwhile, pressured May to share with Parliament her negotiating strategy — something she has steadfastly refused to do, insisting she will not give “a running commentary” on the talks.

“So far May’s team have been all over the place when it comes to prioritizing what is best for Britain, and it’s time they pull their socks up and start taking this seriously,” Liberal Democrat leader Tim Farron said in a statement. 

Pro-E.U. leaders also pointed to what they described as hypocrisy on the Brexit side. One prominent argument for getting out of the E.U. was to restore the sovereignty of Parliament. But in this case, anti-E.U. leaders want Parliament nowhere near a decision that carries huge ramifications for the country’s future.

The court ruling — assuming it is not overturned on appeal — sets up a crucial decision for the 650 representatives in Britain’s House of Commons. Members of the ruling Conservative Party were almost evenly split when the country voted June 23 on whether Britain should stay in the E.U. or leave. But solid majorities of the other major parties in Parliament — including Labour, the Scottish National Party and the Liberal Democrats — all opposed an exit.

May, who took office in July following the resignation of David Cameron, has only a narrow majority in Parliament and could struggle to pass legislation authorizing the start of Britain’s departure.

An early election could be a way for May to regain the initiative. Dominic Raab, a pro-Brexit Tory member of Parliament, alluded to that possibility in an interview with the BBC, effectively daring pro-E.U. parties to force a new vote. “I don’t think those trying to break the verdict of the referendum would be rewarded,” he said in reference to polls that show Conservatives well ahead of their rivals.

Some analysts played down the ruling’s impact, noting that Parliament is unlikely to risk the ire of voters by undermining Brexit.

“We’re moving towards the sovereignty of the people,” said King’s College London historian Vernon Bogdanor, “which is quite a different concept.”

Since taking power, May has often promised that “Brexit means Brexit.” But her government has struggled to put together a coherent strategy for the tough negotiations to come with Europe. 

The talks — set to last two years once Article 50 has been triggered — are likely to focus on the trade-off between Britain’s desire to control E.U. immigration into the country and its wish to retain access to the E.U.’s common market. 

Foreign Secretary Boris Johnson, a leading Brexit proponent, has said the government’s objective “should be having our cake and eating it.”

But European leaders have said that will not be possible and that Britain will have to allow immigrants if it wants to maintain the market access that is at the core of its trading relationships with Europe.

Johnson on Wednesday appeared to make unwitting reference to the government’s struggles, saying in a speech at an awards ceremony sponsored by the conservative Spectator magazine that Britain would make “a titanic success” of Brexit.

George Osborne, Britain’s pro-E.U. former treasury chief, quickly interjected, “It sank.”

Brian Murphy in Washington and Karla Adam in London contributed to this report.