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.
Campaigners argued denying the UK Parliament a vote was undemocratic.
But the government said it already had powers to get talks under way, without the need for consulting MPs and peers.
The judges rejected the case put by ministers by a majority of eight to three.
Supreme Court Brexit ruling: What happens next?
The Supreme Court has dismissed the government’s appeal in a landmark case about Brexit, meaning Parliament will be required to give its approval before official talks on leaving the EU can begin.
The ruling is a significant, although not totally unexpected, setback for Theresa May.
What will the prime minister do next and what impact will the ruling have on the process of leaving the EU, following last year’s referendum vote?
The highest court in England and Wales has dismissed the government’s argument that it has the power to begin official Brexit negotiations with the rest of the EU without Parliament’s prior agreement.
By a margin of eight to three, the 11 justices upheld November’s High Court ruling which stated that it would be unlawful for the government to rely on executive powers known as the royal prerogative to implement the outcome of last year’s referendum.
Under existing rules, the UK must formally notify the EU of its intention to leave by invoking Article 50 of the 2009 Lisbon Treaty. This paves the way for what is expected to be two years of talks about the terms of the UK’s departure and its future relationship with the EU.
Following its four-day hearing in December, the Supreme Court has concluded that Parliament must sanction the triggering of Article 50 before it goes ahead.
The government is expected to respond to the ruling later. The ruling is a headache for ministers although they have had plenty of time to put contingency plans in place.
Will the government challenge the decision?
In theory, Theresa May could take the case to the European Court of Justice, the highest court of the European Union. But from both a logistical and political point of view, there is no chance of this happening.
The prime minister has said she wants to trigger Article 50 by the end of March and won’t be blown off course. A further appeal would drag out the process and mean meeting the timetable would be virtually impossible.
Besides, the government has made great play of the fact that, by leaving the EU, it will end the jurisdiction of the European Court of Justice over the UK and restore the supremacy of British courts.
Asking the European Court of Justice to have the final say over how the UK leaves the EU would be unthinkable to ministers and most Conservative MPs. So the government is likely to accept the ruling and move on.
So Parliament’s voice will be heard – but how?
The Supreme Court case revolved around the principle of parliamentary sovereignty, whether the referendum was advisory or binding and the impact of Brexit on rights conferred by Parliament in 1972, when it passed legislation paving the way for the UK to join the then European Economic Community.
The justices agreed these rights would be affected by Brexit and therefore the Article 50 decision required the explicit authorisation of Parliament.
In short, both the House of Commons and House of Lords will have to vote in favour of it.
What is unclear, at this stage, is what form of parliamentary method the government will use.
Various options have been talked about, including tabling a simple parliamentary motion, amending existing regulations or passing primary legislation – in other words passing a new law to give effect to last year’s referendum decision.
How long will this process take?
Any bill is likely to be short – there has been speculation it might contain just a single clause – and to be given special priority by Parliament, whose order of business is still largely controlled by ministers.
While Tory MPs would like to see it fast-tracked through Parliament, many Labour, Lib Dem and SNP MPs will want as much time as possible to discuss a variety of issues and to make amendments.
However it pans out, Mark D’Arcy says the bill could pass through the Commons before the half-term recess in the middle of February, giving ample time for the Lords to then consider it and for it to become law before the end of March.
While there are some MPs who want the process to be delayed, they are vastly outnumbered by those who want the government to get on with it so that the UK will have left the EU by the time of the next election – scheduled for May 2020.
Is there any chance of Brexit being blocked?
In theory, yes there is. But in reality it is extremely unlikely to happen.
Few, if any, Conservative MPs are likely to vote against Article 50. In fact, only one – the europhile former chancellor Ken Clarke – has said he will do so.
Given that the Tories have a working majority of 15 in the Commons, this means that the bill is guaranteed to pass – especially since a majority of Labour MPs have said they will not stand in the way of the process and many will actually vote for Article 50.
Although the Lib Dems, the SNP and some Labour MPs are likely to vote against, this will make little difference. What will be more interesting is if a coalition of pro-European Conservatives and opposition MPs join forces to win concessions, over the extent of Parliamentary scrutiny of the two-year process.
Events in the Lords – where the government does not have a working majority and there are 178 non-affiliated cross-bench peers – could be more unpredictable. Mark D’Arcy says there are murmurings of an organised attempt to resist Article 50 and a “doomed last stand” by diehard Remainers.
But amid warnings that any attempt to block Brexit could trigger a general election, in which the future of the Lords would be a major issue – it is likely that the skirmishes will amount to just that and the government will eventually get its way.
What does it mean for Scotland, Northern Ireland and Wales?
The Supreme Court case wasn’t just a battle over the powers of the executive and the legislature.
The justices heard a number of separate but related challenges to the government’s Brexit approach, centred around the involvement of the devolved administrations in Scotland, Wales and Northern Ireland.
But the court unanimously ruled that devolved administrations did not need to be consulted.