Photo: AFP
The UK Parliament must vote on whether the government can start the Brexit process, the Supreme Court has ruled.
However, it ruled the Scottish Parliament and Welsh and Northern Ireland assemblies did not need a say.
The judgement means Prime Minister Theresa May cannot start talks with the European Union until MPs and peers give their backing - although this is expected to happen in time for the government's 31 March deadline.
Brexit Secretary David Davis promised a parliamentary bill "within days".
During the Supreme Court hearing, campaigners said 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.
The government argued it already had powers to get talks under way, without consulting MPs and peers. It wants to do this by the end of March.
It said Mrs May could simply use the Royal Prerogative - its executive powers - to invoke Article 50 and begin two years of divorce talks.
The Supreme Court judges rejected the case put by ministers by a majority of eight to three.
Supreme Court President Lord Neuberger said: "The government cannot trigger Article 50 without an act of Parliament authorising it to do so."
The court rejected 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."
Ruling will not delay Brexit - 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."
Attorney General Jeremy Wright said the government was "disappointed", but would "comply" and do "all that is necessary" to implement the judgment of the court, which is the UK's highest judicial body.
- BBC