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.
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 will likely hope that the promise of a “time-limited” government would be enough to secure the support of his critics.
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.
The Labour leader said: “Our priority should be to work together in parliament to prevent a deeply damaging no deal being imposed on the country, denying voters the final say.”
Downing Street has dismissed Mr Corbyn’s plans – claiming he would “overrule the referendum and wreck the economy” if he became prime minister.
The letter was immediately rejected by some of the major figures Mr Corbyn 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.
A Number 10 spokesman retaliated to the Labour leader’s plans by saying: “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.
Some Westminster watchers suspect Number 10 is already planning for an election, fuelled by a Tory MP accidentally posting a screenshot of their emails showing one titled “GE2019” in the subject.
Mr Johnson has previously insisted the “last thing” he wants is to call an election.
He tweeted on Wednesday: “Jeremy Corbyn wants to cancel the referendum and argue about Brexit for years.
“I am committed to leading our country forward and getting Britain out of the EU by 31 October.”