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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.
Commons Speaker John Bercow confirmed MPs would now return on Wednesday.
Supreme Court president Lady Hale said "the effect on the fundamentals of democracy was extreme."
A raft of MPs have now called for the prime minister to resign - 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 and the suspension was far longer than necessary for a Queen's Speech.
Delivering its conclusions, the Supreme Court's president, Lady Hale, said: "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 meant Parliament had effectively not been prorogued - the decision was null and of no effect.
Mr Bercow said MPs needed to return "in light of the explicit judgement", and he had "instructed the House of Commons authorities to prepare... for the resumption of business" from 11:30 BST on Wednesday.
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He said prime minister's questions - which normally takes place on a Wednesday - would not go ahead, though, because Mr Johnson was in New York for a UN summit.
However, Mr Bercow said there would be "full scope" for urgent questions, ministerial statements and applications for emergency debates.
Labour leader Jeremy Corbyn said the ruling showed Mr Johnson's "contempt for democracy", adding: "I invite Boris Johnson, in the historic words, to consider his position."
Mr Corbyn was due to close the Labour Party conference in Brighton with a speech on Wednesday, but has brought it forward to Tuesday afternoon so he can return to Parliament.
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Lawyers for the government had argued the decision to prorogue was one for Parliament, not the courts.
But the justices disagreed, unanimously deciding it was "justiciable", and there was "no doubt that the courts have jurisdiction to decide upon the existence and limits of a prerogative power".
The court also criticised the length of the suspension, with Lady Hale saying it was "impossible for us to conclude, on the evidence which has been put before us, that there was any reason - let alone a good reason - to advise Her Majesty to prorogue Parliament for five weeks".
The damage is done
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 of today's ruling.
What was the court considering?
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.
How did those involved in the case react?
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 Prime Minister Sir John Major - one of the sponsors of the prorogation appeal - said it gave him "no pleasure to be pitted against a government and prime minister of my own party".
"No prime minister must ever treat the monarch or Parliament in this way again."
What about other MPs?
A number of MPs have taken to Twitter to support the court's decision, including former Tory minister Amber Rudd, who resigned her post - and the party whip - over the government's approach to Brexit.
The leader of the Brexit Party, Nigel Farage, said the suspension was the "worst political decision ever" and called for Mr Johnson's chief advisor to 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".
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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."
What happened before Parliament was suspended?
Prorogation is a power that rests with the Queen, carried out by her on the advice of the prime minister.
And at the end of August - shortly before MPs returned from their summer recess - Mr Johnson called Her Majesty to advise she suspend Parliament between 9 September until 14 October.
MPs had been expecting to be in recess for some of these weeks for their party conferences.
But unlike prorogation, a recess must be agreed by a vote, and a number of MPs said they would have voted against it to ensure they could scrutinise Mr Johnson's Brexit plans.
The decision to prorogue prompted an uproar from the Commons, especially from MPs who had planned to take control of Parliament to force through a law to block a no-deal Brexit after Mr Johnson said the UK would leave the EU with or without a deal on the Halloween deadline.
Despite only sitting for a week, they did manage to pass that law ahead of prorogation and it received royal assent on 9 September.
What questions do you have about the Supreme Court's decision?
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https://www.bbc.com/news/uk-politics-49810261
2019-09-24 12:00:03Z
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