Jeremy Corbyn has outlined plans to install himself as a caretaker PM after defeating Mr Johnson in a confidence vote when MPs return from their summer break.
In a letter to fellow Opposition party leaders – and some senior Tory backbenchers committed to thwarting no-deal – he vowed to enter No10 for a “strictly time-limited” period, delay Brexit and trigger a General Election.
He also said he would campaign for a second EU referendum with the option of Remaining in the EU.
The Labour leader wrote: “Following a successful vote of no confidence in the Government, I would then, as Leader of the Opposition, seek the confidence of the House for a strictly time-limited temporary government with the aim of calling a General Election, and securing the necessary extension of Article 50 to do so.
“In that General Election, Labour will be committed to a public vote on the terms of leaving the European Union, including an option to Remain.”
It comes amid persistent fears of a chaotic and damaging no-deal Brexit under the Prime Minister’s “do or die” commitment to leave the EU by the deadline.
But other opposition leaders, notably the Lib Dems Jo Swinson, have previously ruled out taking part in any plan which would result in Jeremy Corbyn becoming PM.
Mr Corbyn wrote: “This Government has no mandate for no-deal, and the 2016 EU referendum provided no mandate for no-deal.
“I therefore intend to table a vote of no confidence at the earliest opportunity when we can be confident of success.
“Following a successful vote of no confidence in the Government, I would then, as Leader of the Opposition, seek the confidence of the house for a strictly time-limited temporary government with the aim of calling a general election, and securing the necessary extension of Article 50 to do so.”
Mr Corbyn said Labour would campaign in that election for a second referendum on EU membership with the option to Remain being available to voters.
Sent to the SNP’s Ian Blackford, Plaid Cymru’s Liz Saville Roberts and Lib Dem leader Jo Swinson, he concluded that he hopes to discuss the proposals with them further to “end the uncertainty and disarray”.
But Lib Dem leader Jo Swinson said Mr Corbyn was “not the person who is going to be able to build an even temporary majority in the House of Commons”.
She added: “I would expect there are people in his own party and indeed the necessary Conservative backbenchers who would be unwilling to support him.
“It is a nonsense.
“This letter is just more red lines that are about him and his position and is not a serious attempt to find the right solution and build a consensus to stop a no-deal Brexit.”
Green leader Caroline Lucas said: We absolutely support and welcome Jeremy Corbyn’s call for a vote of no-confidence in order to avoid the catastrophe of the so-called ‘No Deal’. Preventing a crash-out Brexit on the 31st October was what lay behind my proposal earlier this week.
She added: “I would back a vote of no-confidence if Jeremy Corbyn calls it, but what I ask him to do is to guarantee that if he fails to win the confidence of the House, even for a time-limited temporary Government, that he would commit to supporting an MP who can do that, then deliver the crucial letter to the EU asking for an extension of Article 50, then a People’s Vote.
“After that is the time to hold an election, when voters can make the choice about their representatives to take the UK forward, dealing with the pressing issues within our country.”
A Number 10 spokesperson said: “There is a clear choice: either Jeremy Corbyn as Prime Minister who will overrule the referendum and wreck the economy or Boris Johnson as Prime Minister who will respect the referendum and deliver more money for the NHS and more police on our streets.
“This Government believes the people are the masters and votes should be respected, Jeremy Corbyn believes that the people are the servants and politicians can cancel public votes they don’t like.”
It came as Boris Johnson accused MPs of “collaborating” with Brussels to torpedo Brexit .
The Prime Minister warned British politicians were secretly plotting with EU officials or leaders to scupper the UK’s withdrawal, three years after the country voted to Leave.
The Conservative leader hit out after rebel ex-Chancellor Philip Hammond claimed the PM was poised to “betray” the Brexit result by preparing to quit the bloc without a deal,
Speaking in a “People’s PMQs ” exchange on Facebook , Mr Johnson said Brussels was refusing to reopen the Withdrawal Agreement thrice rejected by the Commons because some in the EU believed MPs would eventually abandon Britain’s departure.
He said: “There’s a terrible kind of collaboration, as it were, going on, between people who think they can block Brexit in Parliament and our European friends.
“And our European friends are not moving in their willingness to compromise, they’re not compromising at all on the Withdrawal Agreement even though it’s been thrown out three times, they’re sticking to every letter, every comma of the Withdrawal Agreement – including the Backstop – because they still think Brexit can be blocked in Parliament.
“The awful thing is the longer that goes on, the more likely it is of course that we will be forced to leave with a no-deal Brexit.
“That’s not what I want, it’s not what we’re aiming for but we need our European friends to compromise.
“The more they think there’s a chance that Brexit can be blocked in Parliament, the more adamant they are in sticking to their position.”
The outburst came after ex-Chancellor Mr Hammond warned the PM was demanding too much from Brussels by calling for the scrapping of the Irish Backstop.
He risked splitting EU nations with the red line and paving the way for departing without a pact, Mr Hammond warned.