LONDON — The British government’s plan for leaving the European Unionwas 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.
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 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 to Parliament a detailed strategy for negotiating the British departure, known as Brexit. She has adamantly resisted doing so, arguing that this might impede her flexibility in the negotiations, which would set the terms of Britain’s relationship with the European Union, and prevent Britain from getting the best possible deal.
Few observers believe that Parliament would go so far as to block a departure from the bloc, as lawmakers themselves voted overwhelmingly to hold the referendum and pledged to abide by the results. But by weakening Mrs. May’s control of the negotiating process, Parliament might soften her declared strategy of controlling immigration, even if that means leaving the European Union’s tariff-free single market — the so-called hard Brexit that was widely perceived as a threat to the British economy.
It was that prospect that led currency traders to bid up the pound 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. If Mrs. May should find parliamentary opposition intolerable, she might ultimately be tempted to call an early general election to gain a wider mandate for Brexit, 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 Brexit.
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 Brexit for now. Yet in the growing environment of constitutional, legal and political uncertainty, the government’s strategy could easily be disrupted.
Mujtaba Rahman, managing director for Europe at the Eurasia Group, a political consulting firm, described the ruling as “a severe setback for Theresa May’s government,” though he added that the government’s timetable could still be met if the Supreme Court rules in its favor.
But if the court upholds the ruling mandating a role for Parliament, the central question will be “what form that role takes: a simple parliamentary motion or actual legislation,” he wrote in an analysis.
If there is a simple vote, “we think the government’s broad negotiating aims and timing for the Article 50 negotiations will not be impacted,” he wrote. But if “legislation is required, the picture will look very different.”
“If May looks likely to lose key votes in the Commons and Lords on her negotiating mandate, the prime minister might be tempted to seek a general election in order to ask the public to endorse her negotiating goals — in effect, to use an election to override Parliament,” Mr. Rahman added.
Although Parliament approved holding the referendum, Mrs. May’s critics argued in court that failing to give lawmakers a voice before applying to leave the European Union would turn them into bystanders as Britain negotiated its disengagement from the bloc after more than four decades of membership. They also pointed out that, technically, the referendum is not legally binding.
The legal challenge revolved around the European Communities Act, a 1972 law that Parliament enacted before Britain joined a forerunner of the European Union in 1973. That act allowed for the incorporation of European law into British law.
Mrs. May and the government argued that they could invoke Article 50 without parliamentary approval by using royal prerogative powers that, in modern times, are exercised by the government in the name of the monarch. The powers include international treaty-making.
But the court found that invoking Article 50 would essentially repeal the 1972 law — and that only Parliament had the power to do so.
“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 court found. “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.”
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.”