The Queen would dismiss Boris Johnson as prime minister if he refused to comply with a law requiring him to seek an extension to Brexit talks, leading Remainer Dominic Grieve has said.

The former attorney general said the PM would be “out in five minutes” if he tried to defy a Supreme Court order to go to Brussels.

Mr Grieve said he expected current attorney Geoffrey Cox and lord chancellor Robert Buckland would resign, while the civil service would withdraw co-operation with a prime minister openly flouting the law in this way.

A cross-party group of anti-no deal MPs passed legislation known as the Benn Act, earlier this month,which requires Mr Johnson to seek an extension to the Article 50 Brexit extension, delaying the date of withdrawal to the end of January, if he has not secured parliamentary approval for a deal or a no-deal outcome by 19 October.

Mr Johnson has branded the legislation a “surrender act” and insists that despite its provisions he will not ask Brussels for an extention beyond the scheduled Brexit date of 31 October.

Supreme Court verdict: Boris Johnson prorogation unlawful Show all 16 1 /16 Supreme Court verdict: Boris Johnson prorogation unlawful Supreme Court verdict: Boris Johnson prorogation unlawful The Supreme Court has ruled against Boris Johnson by declaring his government unlawfully shut down parliament for five weeks EPA Supreme Court verdict: Boris Johnson prorogation unlawful Lady Hale handed down the historic verdict, hailed by opposition MPs and anti-Brexit campaigners. She said all 11 judges were unanimous in deciding that the case is “justiciable”, so the government loses that part of the argument. “The court is bound to conclude therefore that the decision to advise Her Majesty to prorogue was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions.” EPA/Supreme Court Supreme Court verdict: Boris Johnson prorogation unlawful Gina Miller and her team react outside the Supreme Court are the verdict. She said: “Today is not a win for any individual or cause. It is a win for Parliamentary sovereignty, the separation of powers and independence of our British courts. Crucially, today’s ruling confirms that we are a nation governed by the rule of law, laws that everyone, even the Prime Minister, are subject to." PA Supreme Court verdict: Boris Johnson prorogation unlawful A person dressed as a caricature of British Prime Minister Boris Johnson in a prison uniform stands outside the Supreme Court AP Supreme Court verdict: Boris Johnson prorogation unlawful Labour leader Jeremy Corbyn called for Mr Johnson to “consider his position” following the landmark decision, while Lib Dem leader Jo Swinson said he was “not fit to be prime minister” Getty Supreme Court verdict: Boris Johnson prorogation unlawful Scottish National Party (SNP) Westminster leader Ian Blackford raises his arms as he comes outside. He said: "This is an absolutely stunning judgement by the Supreme Court today." He went on to say, "we all want to get back to work, and quite frankly, on the back of this, Boris Johnson must resign immediately." AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Crowds outside celebrated the verdict AP Supreme Court verdict: Boris Johnson prorogation unlawful Delegates at the Labour party conference applaud after hearing the news AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Speaker John Bercow said MPs must now “convene without delay” and confirmed the Commons would return at 11.30am on Wednesday AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Gina Miller said: "“As a result of this judgment, Parliament is open, it was never prorogued. I urge MPs to get back to work immediately.” AP Supreme Court verdict: Boris Johnson prorogation unlawful MPs Anna Soubry, Liz Saville Roberts and Caroline Lucas, together with SNP leader Ian Blackford, react. Green MP Caroline Lucas has said the Supreme Court’s decision is “just the start” Reuters Supreme Court verdict: Boris Johnson prorogation unlawful “The UK deserves a Prime Minister and a Government who act with honesty, integrity and in a manner consistent with our constitution, at all times." AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Anti-Brexit supporters react as they gather outside the Supreme Court in London, Tuesday, Sept. 24, 2019 as it makes it's decision on the legality of Prime Minister Boris Johnson's five-week suspension of Parliament. (AP Photo/Frank Augstein) Frank Augstein AP Supreme Court verdict: Boris Johnson prorogation unlawful The SNP’s Joanna Cherry QC says: “Boris Johnson’s position is untenable and he should have the guts to resign.” PA Supreme Court verdict: Boris Johnson prorogation unlawful epa07865617 Protesters react outside the Supreme Court after the result of a hearing on the prorogation of parliament, in London, Britain, 19 September 2019. The Supreme Court ruled that the suspension of parliament by British Prime Minister Boris Johnson was not lawful. EPA/NEIL HALL NEIL HALL EPA Supreme Court verdict: Boris Johnson prorogation unlawful Tom Tugendhat MP returned to the chamber in the House of Commons within minutes of the Supreme Court ruling PA

But Mr Grieve said that would lead to an instant legal challenge which would reach the Supreme Court within days, where judges would issue a rarely-used order known as “mandamus” requiring a public official to carry out a non-discretionary responsibility.

“He would be taken to court and a writ of mandamus would be issued against him and he would be told that he had, as a matter of law, to write the letter,” Mr Grieve told Sky News.

“I suspect the courts could deal with it very quickly.”

Mr Grieve added: “At that stage, the cabinet secretary and civil service will refuse to work for him. I assume the attorney general and lord chancellor would have resigned, because it is such a flagrant breach of the law.”

Even though Mr Cox has been a vocal supporter of Mr Johnson’s approach to Brexit, Mr Grieve said he was “a good enough lawyer to know you can’t support a prime minister who is breaking the law of the land”.

(EPA)

Pressed on whether Mr Johnson would face further legal action if he refused to comply with the court order, Mr Grieve said: “There is no question of putting the prime minister on trial.

“The Supreme Court - Her Majesty’s judges - telling the prime minister that as a matter of law he has to do something? He will be gone in five minutes. He will be dismissed.”

Asked if it was the Queen who would dismiss Mr Johnson in these circumstances, Mr Grieve replied simply: “Yes.”