Remainers today stepped up their bid to force a second referendum - as Jeremy Corbyn desperately tried to quell a revolt.
Pro-EU MPs are plotting to trigger a crunch Commons vote next weekend on holding a fresh ballot, whether or not Boris Johnson has secured a deal.
They want a referendum to take place before an election, and are increasingly confident the numbers in Parliament are moving their way.
Mr Corbyn again made clear today that he does not favour the idea, appealing for Labour MPs to show 'caution'.
SNP leader Nicola Sturgeon also warned that it would be practically 'impossible' for Remainers to keep a temporary PM in power long enough to push through the necessary legislation.
However, another senior Labour figure, Rebecca Long-Bailey, has signalled that she would now back a referendum.
Jeremy Corbyn again made clear today that he does not favour the idea of a referendum before an election, appealing for Labour MPs to show 'caution'
Nicola Sturgeon (left on the Marr show today) has warned it is probably not 'possible' for a unity PM to push through referendum legislation. Lib Dem leader Jo Swinson (right on Sky News today) said she will never help install Jeremy Corbyn in Downing Street
The PM is planning to summon the Commons to sit on a Saturday next weekend for the first time since the Falklands War.
MPs will either be asked to back his new blueprint - if one has been agreed with the EU - or endorse the UK leaving the bloc without a deal.
Shadow Chancellor John McDonnell gets set to take the reins from 'worn-out' Corbyn
John McDonnell is now effectively the Labour leader as a ‘tired’ Jeremy Corbyn prepares to step back, senior party insiders claimed last night.
They said the Shadow Chancellor is in day-to-day charge of the party with Mr Corbyn happy for his long-term ally to ‘run the show’ and maintain the Left’s grip on the party.
There are even suggestions the Labour leader would formally stand aside to let pro-Remain Mr McDonnell become interim Prime Minister in an anti-No Deal coalition ‘unity government’ if Boris Johnson fails to seal a Brexit deal.
The claims emerged amid reports that Mr McDonnell, one of Mr Corbyn’s oldest political allies, was behind last week’s alleged ‘demotion’ of key Corbyn aide Karie Murphy.
But Remainers are planning to put down an amendment to any motion tabled on so-called 'Super Saturday' that would order a second referendum.
Other attempts to force another ballot have been defeated in the past, but the margin has narrowed over time as the deadlock in Parliament deepens.
Labour's official position is that it wants to win an election, negotiate a new 'sensible' Brexit package, and then hold a referendum in which Remain would be an option.
Mr Corbyn has suggested he would try to stay neutral on whether the UK should leave the bloc, even if he has thrashed out a new Brexit deal.
The veteran left-winger was asked in an appearance on Sky News' Ridge on Sunday programme whether any deal secured by Mr Johnson should be put to a referendum.
'I think many in parliament, not necessarily Labour MPs but others, might be inclined to support it, because they don't really agree with the deal – but I would caution them on this,' he said.
However, Mr Corbyn is increasingly looking to have lost control of party policy on the crucial issue.
His long-time ally John McDonnell said last week that he might back a referendum being held before an election.
And today shadow business secretary Ms Long-Bailey suggested a ballot would be needed in all circumstances.
'I know that many colleagues are of a similar opinion to me,' she told the BBC.
John McDonnell, pictured in Colchester earlier this month, has suggested he would be open to backing a second referendum before a general election
Labour's Hilary Benn (pictured) is among the MPs pushing for a so-called 'People's Vote' to be triggered when the Commons returns for a special Saturday sitting next weekend
'They weren't in favour of a referendum to start off with but now we realise that it has to be put before the people - any final deal - and they have to make the decision as to whether they're happy with that relevant deal or whether they want to remain in the European Union.
'Now that's the case if we're in a general election and Labour renegotiates a deal and equally I think it should be the position if we're faced with a deal that's passed through the Commons by an unelected Tory Prime Minister that could potentially be damaging for our economy.'
However, Scottish First Minister Ms Sturgeon pointed out that holding a referendum would require legislation being pushed through, and take a 'period of time'.
She said that would require a caretaker PM, and there had as yet been no agreement between opposition MPs on who that would be.
'I question whether it will be possible for the opposition parties to come together,' she said.
Underlining the issue, Lib Dem leader Jo Swinson again flatly rejected the idea of Mr Corbyn becoming PM.
After if there were any circumstances in which should could support him, she said: 'No. Jeremy Corbyn is not fit to be PM'