Remainers are bracing to fend off a bid by Jeremy Corbyn to make them choose between No Deal and anointing him Prime Minister.
Labour has been frantically manoeuvring as tensions rise in Westminster with the October 31 Brexit deadline looming.
The country is staring down the barrel of a constitutional crisis next month with Tory rebels threatening to back a no-confidence motion in order to stop Boris Johnson forcing No Deal.
Senior Conservatives including Dominic Grieve have been pushing the idea of installing a respected veteran, possibly Labour's Margaret Beckett, as a 'unity PM' to oversee a solution to the Brexit standoff.
However, there are growing concerns that Mr Corbyn intends to use the crisis to try to force his way into power.
The Labour frontbench this week ruled out supporting a unity government, while shadow chancellor John McDonnell said his plan was to send Jeremy Corbyn to see the Queen and demand to be made PM.
If they stick to the stance, it could mean Remainers being faced with a stark choice in the run-up to Halloween of whether to endorse Mr Corbyn as premier, or accept the UK crashing out of the bloc without a deal.
There are growing concerns that Jeremy Corbyn (pictured left with SNP leader Nicola Sturgeon earlier this year) intends to use the Brexit crisis to try to force his way into power
There is a 14-day window after a no confidence vote for a new PM to be identified, and the Queen is unlikely to ask anyone else to form a government unless it is clear they can command a majority in Parliament.
Otherwise an election is triggered, with many convinced Mr Johnson will simply dissolve the Commons and set the date for after October 31 - stripping MPs of the ability to control the process.
In more signs that Labour is gearing up to try and string together a workable majority, Mr McDonnell made a pitch to the SNP earlier this week by suggesting the party would not block a second independence referendum.
An arrangement with Nicola Sturgeon could add 35 to Labour's tally of 245, excluding two deputy Speakers who do not vote.
Five from Plaid, Green MP Caroline Lucas and other independents could push numbers towards 300.
One obstacle is that the Lib Dems' 13 MPs are hostile to the idea of making Mr Corbyn PM, but along with Tory rebels they too could face a choice between that and No Deal.
Excluding the SNP and Speakers, 320 MPs are needed to have a working Commons majority. However, the bar in a confidence vote could be lower if any anti-No Deal Tories abstain.
One independent MP with close links to Remainers on both sides told MailOnline the Labour leadership did seem to be trying to force them into a choice between No Deal and 'making Jeremy Corbyn PM for a while to sort things out'.
However, they insisted the tactic would not succeed as Mr Corbyn was not widely supported even on his own benches.
The MP also suggested the risk of the Labour leader becoming PM was one of the reasons why Remainers would focus on trying to pass a law forcing Mr Johnson to seek a Brexit extension, rather than voting no confidence to eject him from power.
Countdown to Brexit
Here are some key dates in the countdown to Brexit:
September 3: Parliament returns from its summer recess
Early September: Labour is expected to trigger a vote of no confidence in Boris Johnson's Government
Early/Mid-September: If Boris Johnson loses the confidence vote, Remainers could try to install a cross-party administration to delay Brexit past October 31
Early/Mid September: Alternatively if Mr Johnson loses a general election is triggered if no one can form an administration within 14 days. But there are few rules on when he has to hold it
Early/Mid September: Or the Queen could step in and demand Mr Johnson resign, should he try to remain in No 10, sparking a potential constitutional crisis
October 31: Brexit day, when the UK is currently due to leave the EU
Early November? A possible post-Brexit General Election
Shadow chancellor John McDonnell told an audience at the Edinburgh Fringe yesterday (pictured) his plan is to send Jeremy Corbyn to see the Queen and demand to be made PM
'But once you have pushed the button on a vote of no confidence in HM government you cannot close the lid again.'
Could Corbyn put together a majority?
Excluding the SNP and Speakers, 320 MPs are needed to have a working Commons majority.
The bar in a confidence vote could be lower if any anti-No Deal Tories abstain. But it would still be a tall order for Labour to hit the mark.
Labour MPs: 245
Green Party: 1
Independent Group for Change: 5
Lib Dems: 13
'Then you really are in the deep do-do because the executive has untrammeled control for the key period.'
The MP said even if the Labour front bench staged the confidence vote on the basis that a 'unity PM' would be installed, Mr Corbyn might then not be able to resist the temptation to make a grab for power.
'The Labour leadership only commands about 25 per cent of the PLP. In their heart 75 per cent of the PLP would be up for a government of national unity,' they said.
'But it is a tall order. To get to 300 (MPs) is an awfully big ask.
'It is a much simpler option to find a piece of legislation and get people to vote for that.'
Westminster is braced for an explosive couple of months as the October 31 Brexit deadline looms.
Boris Johnson has made a 'do or die' vow to get the UK out of the bloc by the crucial date, with or without a deal.
But Remainer MPs have been mobilising as they seek ways of blocking the country from crashing out.
There does appear to be a majority in the Commons against No Deal - but MPs are badly split over how they should go about binding the hands of the government.
Some, such as Tory former Cabinet minister Dominic Grieve, have been focusing on whether a no-confidence motion could be passed to evict Mr Johnson from power if he is about to push through Brexit without an agreement.
Mr Grieve has suggested putting a 'unity' premier into No10, perhaps Labour veteran Margaret Beckett, who could ask for an extension to the Article 50 process.
However, Mr Johnson's hard-line Brexit adviser Dominic Cummings has made it known that he would simply refuse to quit even if he loses a confidence vote, and try to call an election for after the deadline.
The other avenue being pursued by Remainers is to pass a law that would oblige the PM to seek and accept an extension to Article 50 from the EU.
Unlike the confidence vote, that would not risk Parliament being dissolved for an election - which could leave MPs powerless to stop No Deal.
But there are concerns that Mr Johnson might either ignore the law or refuse to accept any conditions Brussels puts on an extension.
Option 1: A vote of no confidence in the Government
If the stand-off has not been broken by September, Labour is expected to team up with Tory rebels to stage an early confidence vote to stave off the threat of crashing out.
It is a drastic option that would end the careers of any Conservative MPs who join, but only a PM can request an extension to the Article 50 process, and the legal default currently is that the UK leaves at Halloween with or without an agreement.
As the government's working majority is just one and with strong cross-party opposition to No Deal, there is a serious prospect that Mr Johnson will lose.
But Mr Cummings reportedly 'laughed' at a meeting recently when it was put to him that Mr Johnson would have to quit if he lost such a vote.
Instead, he could try to wait for an election to be triggered and fight it on a 'people vs politicians' ticket, complaining that his opponents are trying to block Brext.
He could also try to fix an election date that was after the Brexit date - robbing the Commons of its ability to control the process and achieving a No Deal Brexit by default.
Remainers believe if the premier refused to go quietly the Queen would be forced to sack him and a unity Government could be installed, with Margaret Beckett a rumoured interim leader
Under the Fixed Term Parliaments Act (FTPA), losing a confidence motion triggers a 14 day countdown to an election being called.
During that period it is possible for a Prime Minister to win a confidence vote and prevent the country going to the polls.
However, the legislation is silent on whether the same premier can return to try again.
Remainer MPs say the Queen would have to sack Mr Johnson if he refused to resign after losing a confidence vote.
But the monarch has always been extremely wary of wading into politics, and it is far from clear that there will be another politician with more chance of commanding a majority in the Commons. Labour has already ruled out the Remainers' favoured option of a national unity government, and Mr Corbyn can barely rely on the support of his own MPs - let along Tories.
Option 2: Passing a law to delay Brexit
Pro-EU MPs have already deployed the tactic of seizing control of Commons business to pass a law insisting on a Brexit delay.
In April a Bill sponsored by Labour's Yvette Cooper and Tory Oliver Letwin made it through Parliament, which paved the way for the Article 50 extension to October 31.
There are moves afoot to cancel the schedule recess next month, which would give more time.
It would require the assistance of Speaker John Bercow, but he has shown himself will to stretch procedural rules to breaking point in order to facilitate MPs getting involved in the Brexit process.
Potentially this approach would involve an emergency debate - known as a Section 24 debate - being tabled and accepted by the Speaker.
He would then allow the MPs to put down a business motion in the slot the next day, setting out the steps for a law to be passed.
That legislation is likely to be very short, simply instructing the PM to seek and accept an extension from the EU.
Remainers are divided over whether the Bill should point the way to a resolution to the Brexit crisis, such as a referendum.
However, if passed by the Commons and the Lords, Mr Johnson would be breaking the law if he refused to ask for an extension.
Refusing to comply would spark legal action, and cause a constitutional crisis.