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 will make a statement to MPs at 12:30 GMT.
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 the government 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 on Brexit.
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.
Outside the 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."
In his statement to MPs later Brexit Secretary David Davis is expected to give more details on the plans to bring forward a Bill to Parliament.
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!"
The government must consult Parliament in order to invoke Article 50 and take Britain out of the European Union, the Supreme Court has ruled.
That means Parliament must take a vote on whether to invoke Article 50, which triggers the process to take Britain out of the EU, before Prime Minister Theresa May, can do so.
May had originally set a timetable to invoke Article 50 in March, but it remains to be seen whether that deadline will now be moved.
However, the judges ruled that the devolved parliaments in Cardiff, Edinburgh, and Belfast do not need to be consulted in order for the prime minister to proceed.
The Supreme Court decision was taken by a majority of eight judges, while three sided with the government.
Delivering the verdict, Lord Neuberger, President of the Supreme Court, said: “In broad terms, Article 50 provides that a country wishing to leave the EU must give a notice in accordance with its own constitutional requirements and that the EU treaties shall cease to apply to that country within two years.”
Noting that the ruling had no bearing on the decision to leave the EU, nor the timetable for such an event, Lord Neuberger said that the issue was only whether the government could trigger Article 50 without first consulting Parliament.
In that respect, the judges had found that “the government cannot trigger Article 50 without Parliament authorising that course”.
The ruling does not overturn the result of the referendum in June, in which the majority of Brits voted to leave the EU, but merely determines the path the government must take towards achieving that end.