LONDON — Britain’s Supreme Court ruled Tuesday that Prime Minister Theresa May must get Parliament to sign off before triggering Britain’s exit from the European Union.
The decision adds a significant hurdle to May’s promise to invoke Article 50 — the never-before-used mechanism for getting out of the E.U. — by the end of March.
Most observers, however, do not expect the decision to derail the Brexit process, set in motion when the country voted for departure by a margin of 52 percent to 48 percent in a June referendum.
Despite most members of Parliament having opposed Brexit, the political costs of blocking it now are seen as high. At most, lawmakers will probably try to shape May’s negotiating strategy rather than attempt to stop her outright.
Once Britain triggers Article 50, it will have two years to negotiate the terms of its departure.
“The British people voted to leave the E.U., and the government will deliver on their verdict — triggering Article 50, as planned, by the end of March,” May’s office said in a statement issued within minutes of the court’s ruling. “Today’s ruling does nothing to change that.”
The Supreme Court’s decision turned on the question of whether the prime minister or Parliament should have the last word in deciding Britain’s status in the European bloc.
Gina Miller, an investment manager, had filed a legal complaint after the referendum, arguing that British lawmakers must be allowed a say on Brexit before the prime minister formally launches the process.
May’s government countered that the principle of “royal prerogative,” enshrined in British law, allows her to decide without Parliament’s consent.
Miller’s view was upheld in a November decision by the London-based High Court for England and Wales. The government appealed the ruling, but the Supreme Court voted 8 to 3 to endorse the High Court’s stance.
Miller celebrated the decision, calling it confirmation that “only Parliament can grant rights to the British people, and only Parliament can take them away. No prime minister, no government, can expect to be unanswerable or unchallenged.”
Ardent Brexit advocates, meanwhile, expressed concern that the court’s decision gives lawmakers room to meddle.
“Today’s judgment gives our out-of-touch establishment the ability to soften or delay the clean Brexit a majority of the British people voted for,” said Arron Banks, a wealthy businessman who helped bankroll the Brexit campaign. “The people have been let down.”
Tuesday’s ruling had been widely expected, and May is now likely to move quickly to try to secure parliamentary approval of her plans.
“Expect some trouble ahead, but not enough to threaten the government’s 31 March deadline,” said Mujtaba Rahman, managing director of the Eurasia Group political consultancy.
Jeremy Corbyn, leader of the opposition Labour Party, said after the ruling that Labour “will not frustrate the process of invoking Article 50.”
But he said the party will make demands in exchange for its support, including that Britain push for access to Europe’s single market, that workers’ rights are respected in any agreement with the E.U. and that Parliament gets a “meaningful vote” at the end of the Brexit negotiations to approve or reject any deal.
Tim Farron, leader of the pro-E.U. Liberal Democrats, went further, demanding a new referendum — “a vote of the people on the final deal.”
The pro-E.U. Scottish National Party — which has the third-most seats of any party in the House of Commons — said it would introduce 50 “serious and substantive” amendments to legislation authorizing the triggering of Article 50.
But May’s Conservative Party — which holds a majority in the House of Commons — is likely to line up solidly behind her.
In a partial victory for May’s government Tuesday, the Supreme Court ruled that the individual states in the United Kingdom — England, Scotland, Wales and Northern Ireland — will not have a veto over Brexit plans.
Scotland and Northern Ireland voted against an E.U. departure. Had the court ruled that the Scottish Parliament and the Northern Irish and Welsh assemblies get their own say in Brexit planning, it could have imposed a far more serious obstacle to May’s plans than any she is now likely to face.
Tuesday’s decision was considered one of the most constitutionally significant for Britain in decades, with important legal precedents regardless of which way the court ruled.
A summary of the judgment was read by the court’s president, David Neuberger, in a short hearing at the court’s neo-Gothic chambers in central London, just across the street from the iconic towers that mark Parliament’s home at Westminster.
May signaled in a speech last week that she intends to seek a clean break from the E.U., with Britain leaving behind Europe’s single market for goods and services and its customs union for trade.
Despite the country being closely divided on the question of whether Britain should get out of the E.U. at all, polls have shown that May’s approach — regarded as a “hard Brexit” — has been well received. Her Conservative Party has a wide lead over the opposition Labour Party, which has struggled to develop a unified position on Brexit.
“While this is a major decision in U.K. constitutional history, the political dynamics may mean that the substantive outcome of the U.K. exit from the E.U. is not altered,” said Andrew Blick, who teaches history at King’s College London.
The November High Court ruling elicited a strongly negative reaction from pro-Brexit politicians and newspapers. The Daily Mail, a leading British tabloid, ran photos of the court’s judges on the next day’s front page along with the headline “Enemies of the People.”
Miller has said she was personally threatened as a result of her role in bringing the Brexit case to the court. She said Tuesday that she was “shocked by the levels of personal abuse that I have received from many quarters over the last seven months, for simply bringing and asking a legitimate question.”