Boris Johnson’s decision to suspend Parliament was unlawful, the Supreme Court has ruled.
Mr Johnson suspended – or prorogued – Parliament for five weeks earlier this month, but the court said it was wrong to stop MPs carrying out duties in the run-up to Brexit on 31 October.
Supreme Court president Lady Hale said “the effect on the fundamentals of democracy was extreme”.
Downing Street said it was “currently processing the verdict”.
Mr Johnson argued he wanted to carry out the prorogation ahead of a Queen’s Speech so he could outline his government’s new policies.
But critics said he was trying to stop MPs from scrutinising his Brexit plans.
the Supreme Court’s president, Lady Hale, said: “The effect on the fundamentals of our democracy was extreme.”
She added: “The decision to advise Her Majesty to prorogue Parliament was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification.”
Lady Hale said the unanimous decision of the 11 justices was that Parliament had not been prorogued – the decision was null and of no effect – and it was for the Speakers of the Commons and Lords to decide what to do next.
Commons Speaker John Bercow welcomed the ruling and said Parliament “must convene without delay”, adding that he would now consult party leaders “as a matter of urgency”.
Wow! This is legal, constitutional and political dynamite.
It is worth just taking a breath and considering that a prime minister of the United Kingdom has been found by the highest court in the land to have acted unlawfully in shutting down the sovereign body in our constitution, Parliament, at a time of national crisis.
The court may have fallen short of saying Boris Johnson had an improper motive of stymieing or frustrating parliamentary scrutiny, but the damage is done, he has been found to have acted unlawfully and stopped Parliament from doing its job without any legal justification.
And the court has quashed both his advice to the Queen and the Order in Council which officially suspended parliament.
That means Parliament was never prorogued and so we assume that MPs are free to re-enter the Commons.
This is the most dramatic example yet of independent judges through the mechanism of judicial review stopping the government in its tracks because what it has done, is unlawful.
Be you ever so mighty, the law is above you – even if you are the prime minister.
Unprecedented, extraordinary, ground breaking – it is difficult to overestimate the constitutional and political significance or today’s ruling.
The ruling was made after a three-day hearing at the Supreme Court last week which dealt with two appeals – one from campaigner and businesswoman Gina Miller, the second from the government.
Mrs Miller was appealing against the English High Court’s decision that the prorogation was “purely political” and not a matter for the courts.
The government was appealing against the ruling by Scotland’s Court of Session that the prorogation was “unlawful” and had been used to “stymie” Parliament.
The court ruled in favour of Mrs Miller’s appeal and against the government’s.
Speaking outside the court, Mrs Miller said the ruling “speaks volumes”.
“This prime minister must open the doors of Parliament tomorrow. MPs must get back and be brave and bold in holding this unscrupulous government to account,” she added.
The SNP’s Joanna Cherry, who led the Scottish case, called for Mr Johnson to resign as a result of the ruling.
“The highest court in the United Kingdom has unanimously found that his advice to prorogue this Parliament, his advice given to Her Majesty the Queen, was unlawful,” she said.
“His position is untenable and he should have the guts, for once, to do the decent thing and resign.”
Former Attorney General Dominic Grieve, who has been an outspoken critic of the suspension, said he was “not surprised” by the judgement because of the “gross misbehaviour by the prime minister”.
He told the BBC’s Victoria Derbyshire programme he was “delighted” the Supreme Court had “stopped this unconstitutional act in its tracks”.
But Tory MP Andrew Bridgen said the court’s decision was “the worst possible outcome for our democracy” and “an absolute disgrace”.
He told the same programme: “What we’ve got is a Parliament that’s completely out of step with sentiment of the country. They’re holding out democracy to ransom.
“What we’re going to see is the Speaker effectively taking control of Parliament and playing to the Remainers’ tune until the 31st of October.”