Parliament must be consulted before the government triggers Article 50, the supreme court has ruled.
The government will now have to bring a bill to parliament before beginning the formal process of leaving the European Union.
Eight of the 11 Supreme Court justices broke in favour of the claimants, arguing that Article 50 and withdrawing from the EU treaties will materially affect laws and rights, and therefore must be authorised by parliament.
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“By a majority of eight to three the Supreme Court rules that the Government cannot trigger Article 50 without an Act of Parliament authorising it to do so," the president of the court, Lord Neuberger, said.
He added: “When the UK withdraws from the EU treaties, a source of UK law will be cut off; further, certain rights enjoyed by UK citizens will be changed. Therefore, the government cannot trigger Article 50 without parliament authorising that course.”
The justices ruled unanimously, however, that the devolved parliaments in Holyrood, Cardiff and Stormont would not get binding votes on Article 50.
Ministers had argued they could invoke the clause by royal prerogative but the high court ruled that Article 50 amounted to a change in law that could only be authorised by parliament, leading to the government’s appeal to the highest court in the UK.
Theresa May has pledged to trigger Article 50 by the end of March, and the government has maintained that its timetable woill not slip whatever the outcome of today’s case.
Ministers are expected to bring forward legislation this week, with some speculation that it could be a single-clause bill.
The bill is unlikely to be defeated in the Commons or Lords, with almost all Tories and the majority of Labour parliamentarians expected to back the bill.
However, Labour leader Jeremy Corbyn has said he will try to amend the bill to safeguard the party’s Brexit priorities in the government’s negotiating strategy.