Jeremy Corbyn plans to block a no-deal Brexit by appealing to Conservative MPs to install him as "temporary" prime minister.
With less than 80 days to go until the 31 October deadline, the Labour leader is urging parties across parliament to oust Boris Johnson in a vote of no confidence.
If he is successful, those who voted against the PM could form a government of national unity.
Mr Corbyn vowed that - if he ascends to power - he will delay Brexit, call a snap general election and campaign for another referendum with the option to Remain.
He likely hopes the promise of a "time-limited" government will be enough to secure the support of his critics.
The Labour leader said he would "immediately" seek an extension to Article 50, saying: "It's hard to say how long that [extention] would be, but obviously long enough to have a general election and for the new parliament to have be able to legislate for the future. "
He added: "I hope that that parliament would legislate so that the people would have a choice between the no-deal Brexit that Boris Johnson is offering or remaining in the EU. But there has to be a popular decision on this."
Mr Corbyn is certain that should Mr Johnson fall to a vote of no confidence that he will take up the reigns of power, saying: "Under all normal parliamentary processes in Britain, the leader of the opposition takes over when the government collapses.
"So that seems to me absolutely in line with all the norms of the unwritten British constitution."
The move comes as MPs plot how to take on Mr Johnson and his pledge to deliver Brexit "do or die" on Halloween.
Downing Street and Brussels are in a stalemate after the new prime minister demanded the EU renegotiates the withdrawal agreement it drew up with Theresa May, which the trading bloc has refused to do.
Mr Corbyn revealed his plan to launch a no-confidence vote "at the earliest opportunity when we can be confident of success" in a letter sent on Wednesday night to opposition party leaders in Westminster and three Tory MPs critical of no deal: Dominic Grieve, Oliver Letwin and Caroline Spelman.
Labour's shadow business secretary Rebecca Long-Bailey claimed it was "constitutionally right" that Mr Corbyn should seek to lead a temporary government, which would not attempt to put in place the party's favoured policies.
She told Sky News: "We're not looking to implement Labour Party policy, that should be for a general election, for the public to determine who the next government should be and we'll campaign quite vigorously on that.
"Jeremy is the leader of Her Majesty's official opposition - the next biggest party in Westminster apart from the government.
"It seems constitutionally right and sensible that Jeremy should lead that caretaker government."
But Mr Corbyn's letter was immediately rejected by some of the major figures the Labour leader hopes - and needs - to win over.
Jo Swinson, leader of the Liberal Democrats, said he was "not the person who is going to be able to build even a temporary majority" in parliament.
She claimed his letter was not a "serious attempt to find the right solution", adding: "It is a nonsense."
Caroline Lucas, co-leader of the Green Party, confirmed she would vote against Mr Johnson in a no-confidence vote.
However, she said holding a general election before a referendum was "the wrong way round" - and warned Mr Corbyn's proposal "does not guarantee that the people are given the final say on Brexit".
Plaid Cymru leader Liz Saville Roberts said it was "disappointing" that Mr Corbyn "cannot bring himself to take the best possible pro-European position".
But Ian Blackford, head of the SNP in Westminster, was more supportive - saying he would be "pleased" to "work together" with Mr Corbyn.
Anna Soubry, the leader of The Independent Group for Change, complained she did not receive the letter, saying that fact and Mr Corbyn's "preference" for an election over a referendum made her believe it was "nothing more than a stunt".
Conservative former minister Alistair Burt was among a group of 21 Tory MPs who this week wrote to Mr Johnson to urge him to steer away from a no-deal Brexit.
Asked if he would support Mr Corbyn as a caretaker prime minister in order to avoid the UK leaving the EU without a deal, Mr Burt told Sky News: "I'm sure there is a very long answer to this but the short answer is no."
Challenged as to how he would respond to MPs who don't want him to become prime minster, Mr Corbyn said: "Well I think they should work with us to ensure there is no to no-deal."
In response to the Labour leader's plans, a Number 10 spokesman 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."
Sky News has also seen a ruling from the UK's most senior civil servant - Cabinet Secretary Mark Sedwill - to a question by Mr Corbyn about whether Mr Johnson could try to force Brexit through by calling an election just after Halloween, legally forcing parliament to shut down.
Mr Sedwill did not rule out the possibility, but added MPs and the EU were in charge of the Brexit date.
Mr Johnson has previously insisted the "last thing" he wants is to call an election.