Theresa May needs the approval of the British parliament to trigger the Brexit process, the UK's most senior judges have ruled.
The British Supreme Court decided that MPs must be given a vote on triggering Article 50, the formal mechanism for leaving the European Union.
However, the court also unanimously ruled that UK ministers are not legally compelled to consult the devolved legislatures in Scotland, Wales and Northern Ireland over the issue.
In a summary of the ruling, Judge David Neuberger explained: "Withdrawal effects a fundamental change by cutting off the source of EU law, as well as changing legal rights.
"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," he added.
On the devolution subject, he said: "The devolution statutes were enacted on the assumption that the UK would be a member of the EU, but they do not require it. Relations with the EU are a matter for the UK Government."
Mrs May has said she intends to invoke Article 50 by the end of March, and her government is now expected to quickly publish a bill in an effort to stick to that timetable.
Although it is thought to be unlikely that the government will lose a parliamentary vote, the court's decision, by an 8-3 majority, could hold up the Brexit process.
British Labour leader Jeremy Corbyn said his party would not "frustrate the process for invoking Article 50", but would try to amend the legislation to stop the UK becoming a "bargain basement tax haven".
The SNP, the third largest party in the Houses of Commons, said it will put forward 50 "serious and substantive" amendments.
The Liberal Democrats, meanwhile, will oppose triggering Article 50 unless Mrs May promises a second referendum on the final exit deal.
The UK's Attorney General Jeremy Wright said the government was "disappointed" with the ruling but would comply with it.
The case was won by a wide-ranging group of anti-Article 50 campaigners, led by investment manager Gina Miller, 51, and hairdresser Deir Dos Santos.
