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.
What the case was about
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.
By Dominc Casciani, BBC home affairs correspondent
This momentous judgement is about one thing alone: the rule of law and how the UK, as a champion of that steady, calm form of government, gets on with the business of leaving the EU.
But what it also makes clear is that membership of the EU is messy in constitutional terms – so only Parliament has the right to pull us out. It can’t be done by the stroke of a minister’s pen.
On the devolution side, the government did however win hands down. The court unanimously ruled that the devolved bodies have no real say in leaving the EU: constitutional power – the means to change the fabric of the United Kingdom, rests with the UK Parliament alone.
What the parties say
Labour leader Jeremy Corbyn said: “Labour respects the result of the referendum and the will of the British people and will not frustrate the process for invoking Article 50.”
But the Scottish National Party said it would put forward 50 “serious and substantive” amendments to the government’s parliamentary bill for triggering Article 50.
Article 50 will begin exit talks with the EU, which are expected to last up to two years.
Liberal Democrat leader Tim Farron said his MPs and peers peers would vote against Article 50 unless there was guarantee of the public having a vote on the final deal reached between the UK and EU.
The case against the government was brought by Ms Miller, an investment manager, and hairdresser Deir Tozetti Dos Santos.
Supreme Court ruling – key points
The 1972 Act that took the UK into the then EEC creates a process by which EU law becomes a source of UK law
So long as that act remains in force, it means that EU law is an “independent and overriding source” of the UK’s legal system
Unless Parliament decides otherwise, this remains the case
Withdrawal from the EU makes a fundamental change to the UK’s constitutional arrangements because it will cut off the source of EU law
Such a fundamental change will be the inevitable effect of a notice being served
The UK constitution requires such changes can only be made by Parliament
The fact that withdrawal from the EU would remove some existing domestic rights of UK residents also renders it impermissible for the government to withdraw from the EU Treaties without prior parliamentary authority