Parliament must vote on whether the government can start the Brexit process, the Supreme Court has ruled.
The judgement means Theresa May cannot begin talks with the EU until MPs and peers give their backing – although this is expected to happen in time for the government’s 31 March deadline.
But the court ruled the Scottish Parliament and Welsh and Northern Ireland assemblies did not need a say.
Brexit Secretary David Davis promised a parliamentary bill “within days”.
What the Supreme Court case was about
During the Supreme Court hearing, campaigners argued that denying the UK Parliament a vote was undemocratic and a breach of long-standing constitutional principles.
They said that triggering Article 50 of the Lisbon Treaty – getting formal exit negotiations with the EU under way – would mean overturning existing UK law, so MPs and peers should decide.
But the government argued that, under the Royal Prerogative (powers handed to ministers by the Crown), it could make this move without the need to consult Parliament.
And it said that MPs had voted overwhelmingly to put the issue in the hands of the British people when they backed the calling of last June’s referendum in which UK voters backed Brexit by 51.9% to 48.1%.
What the court said
Reading out the judgement, Supreme Court President Lord Neuberger said: “By a majority of eight to three, the Supreme Court today rules that the government cannot trigger Article 50 without an act of Parliament authorising it to do so.”
He added: “Withdrawal effects a fundamental change by cutting off the source of EU law, as well as changing legal rights.
“The UK’s constitutional arrangements require such changes to be clearly authorised by Parliament.”
The court also rejected, unanimously, arguments that the Scottish Parliament, Welsh Assembly and Northern Ireland Assembly should get to vote on Article 50 before it is triggered.
Lord Neuberger said: “Relations with the EU are a matter for the UK government.”
Outlining plans to bring in a “straightforward” parliamentary bill on Article 50, Mr Davis told MPs he was “determined” Brexit would go ahead as voted for in last June’s EU membership referendum.
He added: “It’s not about whether the UK should leave the European Union. That decision has already been made by people in the United Kingdom.”
“There can be no turning back,” he said. “The point of no return was passed on 23 June last year.”
Outside the Supreme Court, Attorney General Jeremy Wright said the government was “disappointed” but would “comply” and do “all that is necessary” to implement the court’s judgement.
A Downing Street spokesman said: “The British people voted to leave the EU, and the government will deliver on their verdict – triggering Article 50, as planned, by the end of March. Today’s ruling does nothing to change that.”
Foreign Secretary Boris Johnson, a leading Leave campaigner, tweeted: “Supreme Court has spoken. Now Parliament must deliver will of the people – we will trigger A50 by end of March. Forward we go!”
Investment manager Gina Miller, one of the campaigners who brought the case against the government, said Brexit was “the most divisive issue of a generation”, but added that her victory was “not about politics, but process”.
“I sincerely hope that going forward that people who stand in positions of power and profile are much quicker in condemning those who cross the lines of common decency and mutual respect,” she also said.
Her co-campaigner, hairdresser Deir Tozetti Dos Santos, said: “The court has decided that the rights attaching to our membership of the European Union were given by Parliament and can only be taken away by Parliament.
“This is a victory for democracy and the rule of law. We should all welcome it.”