Former prime minister Sir John Major has said he would be willing to go to court to try and stop Boris Johnson suspending parliament.
He said the move – which Mr Johnson has not ruled out in order to pursue a no-deal exit from the EU if he becomes PM – would drag the Queen into a constitutional crisis.
Sir John, who was in Downing Street from 1990 to 1997, said such a course of action would be “utterly and totally unacceptable”.
He told BBC Radio 4’s Today programme: “In order to close down parliament the prime minister would have to go to Her Majesty the Queen and ask for her permission to prorogue [another term for suspending parliament].
“If her first minister asks for that permission it is almost inconceivable that the Queen will do anything other than grant it.
“She is then in the midst of a constitutional controversy that no serious politician should put the Queen in the middle of.
“If that were to happen there would be a queue of people who would seek judicial review.
“I for one would be prepared to go and seek judicial review.”
Mr Johnson, who is battling Foreign Secretary Jeremy Hunt to take over from Theresa May in Downing Street, has promised to take Britain out of the EU on 31 October “do or die”.
He has expressed confidence about being able to get changes to the Brexit deal Mrs May managed to negotiate with Brussels, but has made clear he would be willing to leave without a deal if necessary.
Mr Johnson’s commitment to the October deadline has raised questions about what he would do if the deadlock at Westminster continues.
A number of votes in parliament have indicated that a majority of MPs are against a no-deal Brexit, because they fear it would cause disruption in numerous areas of British life and hit the economy.
Amid this climate the idea of proroguing parliament – which would bring the current session to an end – has emerged.
The theory goes that if MPs are not sitting, they cannot stop a PM going for no deal.
However, opponents say it would be a startling overreach given Britain is a parliamentary democracy.
Sir John, who is backing Mr Hunt in the leadership race, said: “There is no conceivable justification, wherever we are, in closing down parliament to bypass its sovereignty.
“I seem to recall that the Brexiteers, led by Mr Johnson, actually campaigned in the referendum for the sovereignty of parliament… They can’t be concerned for the sovereignty of parliament except when it is inconvenient to Mr Johnson.”
But critics accused Sir John of hypocrisy, given the fact that he prorogued parliament ahead of the 1997 general election, which stopped a report on the cash for questions scandal being considered by MPs.
He said “we carried the election until almost the very last date” and it was an “absurd charge”.
On a campaign visit to a pub in London Mr Johnson dismissed the comments.
He said: “What we are going to do is deliver Brexit on October 31, which is what I think the people of this country want us to get on and do.
“I think everybody is fed up with delay and I think the idea of now consecrating this decision to the judiciary is really very, very odd indeed.
“What we want is for Parliament to take their responsibilities, get it done as they promised that they would.
“They asked the British people whether they wanted to leave in 2016, the British people returned a very clear verdict so let’s get it done.”
Mr Johnson and Mr Hunt clashed on the issue in a TV debate on Tuesday.
The latter issued a stark warning about the prospect of suspension.
“When that has happened in the past, when parliament has been shut down against its will, we actually had a civil war,” Mr Hunt said.
But Mr Johnson said: “I’m not going to take anything off the table, any more than I’m going to take no-deal off the table.
“I think it’s absolutely bizarre at this stage in the negotiations for the UK – yet again – to be weakening its own position.”