Boris Johnson's decision to suspend Parliament was "unlawful" because it was designed to stop MPs scrutinising the government's Brexit plans, the Supreme Court has ruled.
In a stunning blow for the prime minister that civil service unions warned would add to anxiety in Whitehall, the 11 judges unanimously agreed that his true motivation was to stop parliament from carrying out "its constitutional functions" by being able to hold the government to account.
Delivering the ruling, Supreme Court president Lady Hale said parliament had not, therefore, actually been prorogued.
She said: "The court is bound to conclude that 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 it was now up to the speakers of the House of Commons and House of Lords to decide what to do next.
She added: "Unless there is some parliamentary rule of which we are unaware, they can take immediate steps to enable each House to meet as soon as possible.
"It is not clear to us that any step is needed from the prime minister, but if it is, the court is pleased that his counsel have told the court, he will take all necessary steps to comply with the terms of any declaration made by this court."
Ministers had claimed that the prorogation – which began two weeks ago – was not a matter for the courts and was designed to pave the way for a new legislative programme in a Queen’s Speech on 14 October.
But a cross-party team of MPs and campaigners had argued that Mr Johnson made the move in a bid to close off scrutiny of his Brexit plans by MPs.
The Supreme Court was asked to consider two appeals, with anti-Brexit campaigner Gina Miller challenging a High Court ruling that the government’s decision to prorogue Parliament was outside of its remit.
However, a separate case heard at the Court of Session in Edinburgh found that the prime minister’s advice to the Queen on suspending Parliament had been “unlawful”.
Civil service unions slammed Johnson for his “cavalier” act in suspending parliament.
FDA general secretary Dave Penman said the ruling “can only add to civil servants’ anxiety that they will be asked to break the law, and in turn, the civil service code".
“Concerns about this were already piqued by speculation from Number 10 that it would not implement an act of Parliament, and instead refuse to ask for an extension for Brexit.
"The civil service code instructs civil servants to comply with the law and uphold the ‘administration of justice’. These are values that every civil servant upholds daily.
"The prime minister must now urgently respond to our call to categorically and publicly assure civil servants that they will not be asked to breach these obligations.”
Garry Graham, Prospect deputy general secretary, said “It is clear that the prime minister has acted in a cavalier fashion in terms of the law.
“Civil servants need clear and unambiguous guidance on their responsibilities if in future they are faced with ministerial instructions which they believe may be unlawful or compromise their impartiality.”
He highlighted that Prospect had written to cabinet secretary Sir Mark Sedwill to demand that he ensure ministers and special advisers are fully aware of the civil service code.
“In Prospect's letter to Mark Sedwill we demanded clarity and that is even more vital now,” he said. “To date we have received no response to our letter whilst at the same time there is increasing concern amongst civil servants.”
PCS general secretary Mark Serwotka said: "This judgement shows that we need a general election as a matter of urgency. Boris Johnson has put himself outside the law by illegally shutting down parliament. We have a PM with no democratic mandate, no majority and now our highest court has found against him."
Johnson, speaking in New York where he is attending the UN general assembly, yesterday insisted he had been right to prorogue parliament – and he would not be forced to resign over the row.
He said: "It is absolutely right to be having a Queen's Speech setting out what exactly should be done, setting out what we want to do on crime, on the health service, on education.
"When it comes to parliamentary scrutiny, what are we losing? Four or five days of parliamentary scrutiny when parliament has had three years to discuss issues and will be able to come back and discuss Brexit after the European Council on October 17-18.
"It is absolutely nonsense to say there will be no parliamentary scrutiny."
Responding to the court's ruling, House of Commons speaker John Bercow said: "I welcome the Supreme Court’s judgement that the prorogation of Parliament was unlawful.
"The judges have rejected the government’s claim that closing down parliament for five weeks was merely standard practice to allow for a new Queen’s Speech.
"In reaching their conclusion, they have vindicated the right and duty of parliament to meet at this crucial time to scrutinise the executive and hold ministers to account. As the embodiment of our parliamentary democracy, the House of Commons must convene without delay. To this end, I will now consult the party leaders as a matter of urgency."