Boris Johnson faces ‘gravest constitutional crisis since Charles I’ if he defies confidence vote

Boris Johnson has been warned he could trigger the “gravest” constitutional crisis since Charles I if he refused to resign after losing a vote of no confidence, as a prominent businesswoman threatened a judicial review.

Former Conservative foreign secretary Sir Malcolm Rifkind’s remarks came amid suggestions the new prime minister could defy convention and remain in Downing Street despite losing a vote – potentially dragging the monarch into the row.

Dominic Grieve, a prominent pro-EU Tory MP, also said constitutional convention cannot be ignored by Mr Johnson, claiming the Queen “is not a decorative extra” and may be forced to “dispense with his services herself”.

It is expected Labour leader Jeremy Corbyn will table a motion of no confidence in the autumn to prevent a no-deal Brexit, and some senior Tory MPs have suggested they could vote against Mr Johnson’s new government as a last resort.

Sir Malcolm, who served in Sir John Major’s government, wrote in a letter to The Times: “It is a convention, and not part of any act, that the government has to resign if defeated in a confidence vote, and that a general election is the ultimate way in which a constitutional crisis can be resolved.”

But he added that the Supreme Court “cannot enforce respect for conventions” that are not incorporated in an act of parliament.

“It is, however, also ‘only’ a convention, not a law, that the Queen can act only on the advice of her ministers,” he said. “It is ‘only’ a convention that the Queen cannot dismiss her prime minister even if he is flouting the convention.”

He continued: “If the prime minister refused to respect the normal consequence of losing a confidence vote, and if he sought to prevent both parliament and the electorate having a final say on no deal, he would create the gravest constitutional crisis since the actions of Charles I led to the civil war.

“I have great confidence that the prime minister will ignore the advice of Dominic Cummings. King Charles lost his head by flouting the constitution. Mr Johnson will wish to keep his, while some around him are, clearly, losing theirs.”

Sir Malcom’s remarks came as high-profile businesswoman and campaigner Gina Miller, who previously launched a successful bid against Theresa May’s government over Article 50, claimed it would be “unlawful” if Mr Johnson refused to step down after losing a confidence motion.

Ms Miller said the Fixed-term Parliaments Act is “governed by convention”, and told Radio 4’s Today programme: “There is solid convention that a prime minister losing a vote of no confidence must step down.”

She added: “While the Fixed-term Parliaments Act does not replace convention, it can be said the frustration of the principle – which is actually what we invoked in my first case and we won – would be in play because Prime Minister Johnson’s refusal to go would frustrate the operation and the purpose of the act, and therefore be unlawful.”

Asked if she could be granted a judicial review in time, Ms Miller replied: “I have already instructed my legal team to take whatever steps necessary to ensure that a prime minister doesn’t attempt to put themselves above the law, and that we would seek some judicial review and clarity. That is already in motion and we would be ready.”