British Prime Minister Boris Johnson's controversial five-week suspension of Parliament was unlawful, the UK Supreme Court has ruled, in a landmark decision that will have far-reaching constitutional implications.
Judges unanimously upheld an earlier decision by Scotland's highest civil court, which ruled that Johnson had acted illegally to suppress parliamentary scrutiny of his Brexit strategy.
Supreme Court President Lady Hale said Johnson's advice to the Queen 'was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification.'
Extraordinarily, she added that the prorogation was 'void and of no effect,' and that 'Parliament has not been prorogued.'
Huge defeat for PM
The decision is a huge defeat for the Prime Minister, and will likely set off a bitter argument over whether the justices have strayed too far into the UK's political arena.
But, speaking in New York where he is attending the United Nations General Assembly (UNGA), Johnson was defiant, saying that while he has 'the utmost respect for our judiciary ... I don't agree with the verdict.'
Johnson said the government would respect the ruling, adding that the 'most important thing' is that the UK leaves the EU on October 31, 'come what may.'
A government source told CNN the Prime Minister would fly back to the UK on Tuesday night, after his speech at the UNGA.
Following the judgment, John Bercow, the Speaker of the House of Commons, said Parliament would resume on Wednesday from 11:30 a.m. local time (6:30 a.m. ET).
It is not yet clear if Johnson plans to address MPs on Wednesday, following his return from the US.
Bercow said there would be no Prime Minister's Questions session (normally held on Wednesdays) but that there would be opportunities for urgent questions, ministerial statements and emergency debate applications.
The House of Lords, Parliament's upper house, will also resume on Wednesday, at 3 p.m. local time (10 a.m. ET).
Johnson's decision to suspend parliament until mid-October was hugely contentious. Opposition lawmakers accused him of trying to shut down political efforts to stop a no-deal Brexit on October 31.
The government insisted the prorogation was constitutional, and normal procedure when a government wants to restart the parliamentary session with a new legislative agenda.
READ LADY HALE'S FULL SUMMARY OF THE COURT'S DECISION HERE:
UK opposition leader Jeremy Corbyn called for Johnson to reconsider his position 'and become the shortest serving Prime Minister that has ever been.'
Speaking at the Labour conference in Brighton, Corbyn added that Johnson must 'obey the law, take no deal off the table and have an election to elect a government that respects democracy that respects the rule of law and brings power back to the people, not usurps it.'
Tuesday's decision follows last week's high-stakes hearing in the Supreme Court, at which the justices were asked to decide between competing decisions of Scottish and English courts.
They upheld a case brought in Edinburgh by a cross-party group of more than 70 lawmakers led by Scottish National Party MP Joanna Cherry. They agreed with the Court of Session that Johnson's suspension of Parliament was motivated by the 'improper purpose of stymying Parliament.'
At the same time, they overturned a ruling by the High Court in London, which found against the anti-Brexit campaigner Gina Miller. The High Court had earlier ruled that the decision to suspend Parliament was a political matter, and not one that could be reviewed by the courts.
Miller's legal action was joined by John Major, treating Britain to the extraordinary spectacle of a former Conservative prime minister suing his successor.
Speaking outside court following the ruling, Cherry told reporters that she was 'absolutely delighted that the UK Supreme Court has agreed with Scotland's Supreme Court.'
She added: 'This is a huge victory for the rule of law for democracy and it's very much in keeping with Scottish constitutional traditions that neither of the governments nor indeed the monarch are above the law.'
Meanwhile, Brexit coordinator for the European Parliament Guy Verhofstadt tweeted that 'the rule of law in the UK is alive and kicking' after the decision.
'Parliaments should never be silenced in a real democracy,' he added. 'I never want to hear Boris Johnson or any other Brexiteer say again that the European Union is undemocratic.'
This story has been updated to clarify what the ruling said about the reasons Boris Johnson gave to Queen Elizabeth II for the prorogation of Parliament.