LONDON — Britain's High Court brought government plans for leaving the European Union screeching to a halt Thursday, ruling that the prime minister can't trigger the U.K.'s exit from the bloc without approval from Parliament.
The government said it would go to the Supreme Court to challenge the ruling, which has major constitutional as well as practical implications.
The pound, which has lost about a fifth of its value since the June decision to leave, shot up on the verdict, rising 1.1 percent to $1.2430.
Britons voted by a margin of 52 to 48 percent to leave the EU, a process known as "Brexit." Several claimants challenged the plans for Brexit in a case hinging on the balance of power between Parliament and the government.
Prime Minister Theresa May has said she will launch exit negotiations with the EU by March 31. She is relying on a power called the royal prerogative that lets the government withdraw from international treaties.
Claimants argue that leaving the EU will remove rights, including free movement within the bloc, and say that can't be done without Parliament's approval.
Three senior judges ruled that "the government does not have the power under the Crown's prerogative" to trigger the official exit process.
The British government immediately said it would appeal the judgment. The government 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 to hear the appeal before the end of the year.
The case is considered the most important constitutional matter in a generation.
Underscoring the importance of the case, May put Attorney General Jeremy Wright in charge of the legal team fighting the claim. Wright argued that the lawsuit was an attempt to put a legal obstacle in the way of enacting the result of the EU referendum.
May wants to use royal prerogative, historic powers officially held by the monarch, to trigger Article 50 of the EU's treaty, which starts two years of talks before Britain's departure from the EU. The powers, which have in reality passed to politicians, enable decisions to be made without a vote of Parliament and cover matters as grave as declaring war or as basic as issuing passports.
Historically, royal prerogative has also applied to foreign affairs and the negotiation of treaties.
Financial entrepreneur Gina Miller, a lead claimant in the case, said the result "is about all of us ... It's about our United Kingdom and all our futures."
She backed the losing "remain" side in the EU referendum, but has said the lawsuit is not an attempt to stop Brexit — just to ensure that Parliament is sovereign.
Still, the pound's rise signaled that the ruling boosted the hopes of the financial sector, which is largely opposed to Brexit.
Many in the markets hope the judgment will delay the Brexit process or diminish the government's ability to push through a so-called "hard Brexit," which would see Britain leave the European single market. The hope is that lawmakers won't give their backing if the government intends to push for that sort of deal.
The ruling angered pro-Brexit campaigners, who fear politicians might try 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."
David Greene, lawyer for claimant Deir Santos, said the government must accept the "the constitutional reality that Parliament must have early involvement in the process."
Greene, a senior partner at the firm Edwin Coe, says "democracy has been reaffirmed and now very much needs to show it is alive and kicking."
Andrew Kidd, a lawyer who wasn't involved in the case, said the ruling was a resounding victory for a claim that "was seen as a longshot."
He said the claimants had made "very clear, cogent arguments," while the government's case was "kind of like a shotgun firing pellets, hoping one would hit the target."
Associated Press writer Pan Pylas contributed to this story.