Theresa May needs the approval of Parliament to trigger the Brexit process, Britain’s most senior judges have ruled.
The Supreme Court has decided that MPs must be given a vote on triggering Article 50, the formal mechanism for leaving the European Union.
Despite the setback, Downing Street insisted Mrs May’s plan to begin negotiations on leaving the EU by the end of March remains on track.
Revealing the decision, Supreme Court President Lord Neuberger said: “The referendum is of great political significance, but the Act of Parliament which established it did not say what should happen as a result.
“So any change in the law to give effect to the referendum must be made in the only way permitted by the UK constitution, namely by an Act of Parliament.
“To proceed otherwise would be a breach of settled constitutional principles stretching back many centuries.”