LONDON (AP) — Britain's High Court brought government plans for leaving the European Union to a screeching halt Thursday, ruling that the prime minister can't trigger the U.K.'s exit from the 28-nation bloc without a vote in Parliament.
The case is considered the most important constitutional matter in a generation. The Conservative government said it would go to the country's Supreme Court to challenge the ruling, which, if upheld, could prevent it from starting exit talks with the EU by March 31 as planned.
The British pound, which has lost about a fifth of its value since the June 23 vote to leave the EU, shot back up after Thursday's verdict, rising more than 1 percent to $1.2493 before falling back slightly.
Britons voted by a margin of 52 to 48 percent to exit the EU, a process known as "Brexit." Prime Minister Theresa May has said she will invoke Article 50 of the EU treaty, which launches two years of exit negotiations, by the end of March.
Several claimants, including a hairdresser and a financial entrepreneur, challenged May's right to trigger Brexit, in a case with major constitutional implications that hinges on the balance of power between Parliament and the government.
May wants to use royal prerogative, historic powers officially held by the monarch, to trigger Article 50. The powers, which have in reality passed onto politicians, enable decisions about issues including international treaties to be made without a vote of Parliament.
The claimants argued that leaving the EU will remove legal rights held by British citizens — including free movement and job opportunities within the EU — and that can't be done without Parliament's approval.
Three senior judges agreed, ruling that "the government does not have the power under the Crown's prerogative to give notice pursuant to Article 50 for the U.K. to withdraw from the European Union."
The judges backed the claimants' argument that "the Crown could not change domestic law and nullify rights under the law unless Parliament had conferred upon the Crown authority to do so."
The British government immediately said it would appeal. It said in a statement that Britons voted to leave the bloc in a referendum approved by an Act of Parliament, "and the government is determined to respect the result of the referendum."
The Supreme Court has set aside time in early December to hear the appeal.
Nick Barber, associate professor of constitutional law at Oxford University, said the court had ruled decisively that "you can't use executive power to overturn statutory rights."
"It's a very strong judgment. It's a unanimous judgment and it's a judgment by three of the most senior judges at that level you could hope to get," Barber said. "So if I was the government, I wouldn't be optimistic about the Supreme Court reaching a different view."
There is a small chance the Supreme Court could refer the case to the European Court of Justice, the EU's highest court, if it thinks a legal principle needs clarification, but Barber said that's extremely unlikely.
Supporters of Brexit have called the lawsuit an attempt to put legal obstacles in the way of enacting the result of the EU referendum.
But financial entrepreneur Gina Miller, a claimant in the case, said it wasn't an attempt to stop Brexit but a move to ensure that Parliament is sovereign.
"I hope the MPs will do their job and debate this in a sober, grown-up way," she said.
David Greene, lawyer for hairdresser Deir Santos, another claimant, said the government must accept the "the constitutional reality that Parliament must have early involvement in the process."
"Democracy has been reaffirmed and now very much needs to show it is alive and kicking," said Greene, a senior partner at the firm Edwin Coe.
It's unlikely the ruling will stop Britain from leaving the EU eventually. Most lawmakers accept that voters' choice must be respected — but they differ widely on what form Brexit should take.
Pro-EU legislators hope the ruling will force the government to set out its plans for exit negotiations before triggering Article 50, something May has previously ruled out.
A majority of members of Parliament backed the "remain" side in the referendum, but could be willing to support the start of exit talks if it's clear that the government won't seek a "hard Brexit," in which Britain leaves the EU's single market of 500 million people.
The pound's rise signaled that the ruling boosted the hopes of the financial sector, which is largely opposed to Brexit.
The ruling infuriated pro-Brexit campaigners, who fear an attempt to block or delay Britain's EU exit. U.K. Independence Party leader Nigel Farage, who helped lead the campaign against the EU, tweeted: "I worry that a betrayal may be near at hand."
"I now fear that every attempt will be made to block or delay the triggering of Article 50," Farage said. "If this is so, they have no idea of the level of public anger they will provoke."
Across the English Channel, EU Commission spokesman Margaritis Schinas said it "is not for us to comment on" the British court decision.