LONDON — Prime Minister Theresa May must secure the approval of Parliament before she can begin the process of taking Britain out of the European Union, the Supreme Court of the United Kingdom ruled Tuesday.
The ruling, which upholds an earlier decision by the High Court in London, creates another hurdle for May, who has promised to begin a two-year, irreversible process of exit negotiations by the end of March by invoking the EU’s Article 50, the legal mechanism for leaving the bloc.
In its ruling, the court noted that Parliament had approved the 1972 legislation that enabled the country to join the EU and incorporated European law into British law. Leaving the bloc would take away from British citizens a number of rights that had been granted by the bloc.
As a result, “the government cannot trigger Article 50 without an act of Parliament authorizing that course,” David Neuberger, the Supreme Court president, said in announcing the decision, which was approved 8 to 3.
Although a majority of lawmakers had campaigned to stay in the EU before the referendum last year, most political observers said it was unlikely that legislators would reject the will of the voters.
“There’s no going back,” David Davis, the British official tasked with overseeing the withdrawal, told Parliament later Tuesday. “The point of no return was June 23,” he said, referring to the date of the referendum.
In one important victory for the government, the court ruled that May would not need separate approval from the regional legislatures in Northern Ireland, Scotland, and Wales.
“The devolution statutes were enacted on the assumption that the UK would be a member of the EU but they do not require it,” Neuberger said. “Relations with the EU are a matter for the UK government.”
Within minutes of the ruling, which was not unexpected, Jeremy Wright, Britain’s attorney general, said outside the Supreme Court building in London that the government was disappointed with the decision but that it would comply with it.
May’s office said that the ruling would not affect the timetable to begin talks on a withdrawal, and Davis said that the government would soon introduce a simple bill to invoke Article 50.
The Conservatives could also take heart in the section of the ruling on the regional legislatures, which Theresa Villiers, a member of Parliament for the party, described as “very good news.”
Clearly disappointed that Parliament in Scotland, where there is a resilient independence movement, would not get its own vote on invoking Article 50, the Scottish National Party signaled its intention of preventing the national government from doing so.
In a statement, the party said it would submit “50 serious and substantive” amendments to any legislation that would allow the withdrawal talks to proceed.
The case has underscored the polarizing nature of the June referendum, in which 52 percent of Britons voted to leave the EU.
One of the plaintiffs, Gina Miller, an investment fund manager, has said she was threatened with murder and rape by supporters of Brexit, as the process of leaving the bloc is known, who have accused her of trying to sabotage an exit. A lawyer by training, Miller has said she was merely standing up for the rights of Parliament.
“This case was about the legal process, not about politics,” Miller said in a news conference outside the Supreme Court.
Miller said she had been the victim of a torrent of criticism since filing the case, and said she was “shocked by the levels of personal abuse that I have received from many quarters over the last several months for simply bringing and asking a legitimate question.”
Even the judges have found themselves under immense pressure. Members of the High Court who ruled against the government in November, setting the stage for the Supreme Court decision, were described by one tabloid newspaper as “enemies of the people.”
The ruling comes a week after May first outlined her vision for a clean break with the EU single market, one in which Britain would close its borders to visa-free migration for citizens from other countries in the bloc — a “hard Brexit” as it is often referred to here.
Under the terms of Article 50, the prime minister, who officially supported remaining in the bloc but expressed only tepid support for it during the referendum campaign, has two years to complete exit negotiations once she invokes the rule.
In her speech last week, May pledged to give both houses of Parliament the opportunity to vote on any accord, but lawmakers from across the political spectrum have made clear that they want to be involved from the start.
“I and many others did not exercise our vote in the referendum so as to restore the sovereignty of this Parliament only to see what we regarded as the tyranny of the European Union replaced by that of a government,” Stephen Phillips, a member of May’s Conservative Party, said when the case was first brought.
If no deal is reached with the 27 other member states once the two-year countdown begins — or if Parliament rejects a final deal at that stage — it would most likely result in the sort of “cliff edge” breakup that May, along with Britain’s bankers and business leaders, hoped to avoid.
May’s government had argued that under Britain’s unwritten constitution, the leadership can make or leave international treaties without parliamentary approval.
The High Court disagreed, ruling in November that it would be unlawful to use executive powers known as “royal prerogative” to start a process that would take rights away from citizens that had been granted by Parliament. Only Parliament could take those rights away, the court said.
The government appealed the decision, taking the case to the Supreme Court, the country’s highest judicial body, which came to its own conclusion after holding four days of hearings in front of its 11 justices last month.
Most legal commentators had predicted that the government would lose the case, and according to British news reports, officials have already started drafting parts of a bill that would be subject to parliamentary approval in a bid to keep to the March timetable.