Sir Malcolm Rifkind warns Boris Johnson refusal to quit after no confidence vote would spark constitutional crisis

Boris Johnson has been warned that refusing to quit after losing a vote of no confidence would trigger the "gravest" constitutional crisis since the English Civil War.

Conservative former Foreign Secretary Sir Malcolm Rifkind used a letter to The Times to speak out following claims that the Prime Minister could seek to stay in office even if the House of Commons brings him down in a bid to thwart a no-deal Brexit.

Johnson's top Number 10 adviser Dominic Cummings is reported to have told colleagues that the PM would refuse to quit if Conservative Remain-supporters teamed up with Labour to bring down the Government.

But Sir Malcolm warned Downing Street that such a move risked "flouting the constitution".

"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," the Tory grandee - who has declared his own support for a second Brexit referendum - said.

He added: "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."

The comments came as former attorney general Dominic Grieve - who has repeatedly clashed with Cummings over Brexit - also hit out at the idea of Johnson staying in Number 10 after losing a confidence vote.

He told the BBC: "To argue that if you lose a vote of no confidence you will simply sit it out and barricade yourself in Downing Street...is simply breathtaking, stupid, infantile, and it won’t work.”

The Conservative backbencher has instead talked up the idea of an anti-Brexit government of national unity taking over if Johnson loses - but that idea was dismissed by Labour's Rebecca Long-Bailey on Tuesday night.

"What we don’t want is a national unity government that gives Boris some sort of get out of jail free card, so as soon as Brexit’s been sorted out, he can sail back in without any problems at all, without a sufficient Parliamentary majority," the Shadow Business Secretary warned.

Meanwhile, Scottish Tory leader Ruth Davidson has re-opened her feud with Johnson by insisting that "millions" of people could not afford the economic shock of no-deal.

Speaking at the Edinburgh Fringe, she also warned that the Prime Minister has no mandate to take the UK out of the EU without a deal.

She said: "My own view is that, particularly after the issues that we had in 2008 with the worldwide economic crash, there’s quite a lot of people that would really struggle, even if there was a very mild economic shock to the United Kingdom, even if it was only a short-term one.

“There are millions of people in this country who have no savings, whose wages haven’t kept up and overtaken inflation in the last 10 years, who live two or three weeks out of every month in their overdraft as it is, and actually can’t afford an economic shock to this country.

“That’s one of the reasons I’m urging people to get round the table and have a compromise and make sure we get a deal across the line.”