Updated Nov. 3, 2016 6:49 a.m. ET
LONDON—A U.K. court ruled Thursday that the government can’t trigger negotiations to leave the European Union without a vote from Parliament, dealing a major blow to Prime Minister Theresa May’s plans.
The U.K. government said it was disappointed by the judgment and that it would appeal the ruling, which could force the government to get parliamentary approval to trigger Article 50, the formal mechanism that begins a two-year window for exit negotiations.
If Parliament is allowed to vote on the trigger, lawmakers could influence her approach to Brexit and if a majority is opposed it could theoretically delay or even stop the process. Mrs. May’s ruling Conservative Party is the largest party in Parliament, with a majority of 15 seats.
The pound surged more than 1% against the dollar following the decision.
“The country voted to leave the European Union in a referendum approved by act of Parliament,” a U.K. government spokesman said. “And the government is determined to respect the result of the referendum.”
The Supreme Court is on standby to give an expedited hearing before a full bench of 11 judges early next month.
The case—brought by a group of British citizens with the help of some of the U.K.’s top constitutional lawyers—is a major complication for Mrs. May, who has said she plans to invoke Article 50 by the end of March. Mrs. May has taken a hard line on the terms she would seek in talks, suggesting she would prioritize the right to curb immigration over access to the EU’s tariff-free single market.
Some political experts have suggested that if the plaintiffs win, pro-EU lawmakers will have an opportunity to steer the country toward a “softer” exit, with more ties to the bloc and a more open immigration policy.
Nigel Farage, the interim leader of the euroskeptic UK Independence Party and a prominent Brexit campaigner, said he feared the political class may now try to block or delay the triggering of Article 50 and warned, “If this is so, they have no idea of the level of public anger they will provoke.”
The case, which combines at least seven private actions brought by individuals who supported Britain’s continued EU membership, underscores the complexities involved in the decision to leave the 28-member union, as it becomes the first member of the modern EU to do so.
The government says it has the right to leave because of the so-called royal prerogative, in which executive authority is given to ministers so they can govern on the monarch’s behalf. Lawyers representing the government also say Mrs. May has a responsibility to carry out the wishes of the people as expressed in the June vote to break away.
But the claimants argue that triggering Article 50 without parliamentary consent would effectively override a 1972 statute that enshrines European law in the U.K. and which they say ensures rights that can only be removed by Parliament.
“We decide that the government does not have power under the Crown’s prerogative to give notice pursuant to Article 50 for the U.K. to withdraw from the European Union,” said the chief judge, Roger Thomas.
Spearheading the legal challenge are British businesswoman Gina Miller and hairdresser Deir Dos Santos. Grahame Pigney, a France-based expatriate who used crowdfunding from more than 4,000 people to pay for lawyers, has also joined the suit as a co-party. Lawyers representing the Scottish and Welsh government are also participating in the case as observers.
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