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 parliamentary approval.
The government said it would go to the Supreme Court to challenge the ruling, which if upheld could prevent it starting exit talks by March 31 as planned.
The pound, which has lost about a fifth of its value since the June 23 decision to leave the EU, shot back up on the verdict, rising 1.1 percent to $1.2430.
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, launching 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. They argued that leaving the EU will remove rights, including free movement within the bloc, 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 the judgment. 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 to hear the appeal before the end of the year.
The case is considered the most important constitutional matter in a generation.