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.
But the government said it already had the powers to trigger Article 50 of the Lisbon Treaty – getting talks under way – without the need for consulting MPs and peers. It wants to do this by the end of March.
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.”
The court also 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.”
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.