Boris Johnson is way short of a majority in the Commons since he stripped the whip from 21 Tories who rebelled over No Deal Brexit – and Amber Rudd then quit in solidarity.
Mr Johnson challenged critics to table a vote of no confidence and face him in an election during stormy Commons clashes last week.
However, up until now opposition parties have refused to take up the offer – saying they want to wait until the Halloween Brexit deadline has been pushed back.
A Remainer law passed earlier this month obliges Mr Johnson to beg the EU for an extension if an agreement has not been reached by October 19.
Under the Fixed Term Parliaments Act, when a PM loses such a battle there is a two-week period for someone else to win a confidence vote. If that does not happen an immediate election is triggered.
There have been fears that a no confidence vote could put Mr Johnson in control of the Brexit timetable, as he would be able to dictate the date for a general election if another PM who can secure a Commons majority does not emerge within a fortnight.
Here are the scenarios for what happens after the government loses a confidence vote:
Jeremy Corbyn secures a majority
Parliament’s Remainers decide that Mr Corbyn is their only option, making him best placed to succeed as PM.
As the numbers are on a knife-edge, a vote is called and Mr Corbyn scrapes through. All 245 Labour MPs swallow their doubts and endorse him as PM.
They are joined in the division lobbies by the SNP’s 35 MPs, and 18 Lib Dems MPs.
He picks up another 22 votes to take him over the 320 mark from a variety of sources. The four Plaid and one Green MP are relatively kindly disposed.
But the five-strong Independent Group led by former Tory minister Anna Soubry, and 32 independent MPs will be harder work. Ex-Labour MPs John Mann, a strong critic of the leader over anti-Semitism, and Brexit supporter Frank Field, are incredibly unlikely to come over.
It is possible Mr Corbyn could pick up a few supporters from the 22 former Tory Remainer rebels. Ex-Chancellor Ken Clarke has indicated that in extremis he could tolerate a short-lived Corbyn premiership.
Mr Corbyn goes to Brussels and secures a Brexit extension until January 31, then calls an election – which he is able to fight with the advantage of being ensconced in Downing Street.
The SNP appears to be on board with a Corbyn ‘caretaker’ government – but as this chart shows he will need many more MPs to fall into line to get himself over the winning line of 320 votes and into Downing Street. Mr Corbyn could need to pick up seven of the 22 Tory Remainer rebels stripped of the whip, and persuade half the 10 other independent politicians
Another ‘unity PM’ takes over
After Mr Johnson is defeated in an initial confidence vote, Mr Corbyn tries to put together a majority. But it soon becomes clear that he cannot get close to the numbers needed – as even some MPs in his own party will not tolerate him as PM.
Instead, under huge pressure from his own shadow cabinet, Mr Corbyn agrees to support a less controversial candidate as a temporary leader for the country.
Labour veteran Margaret Beckett and Tory grandee Ken Clarke have both been touted as potential candidates – partly because they are considered too old to want to stay around as premier for long.
No10 aides believe Amber Rudd has been positioning herself as a Chancellor in a Beckett administration.
The opposition parties mass behind the new PM, who comes into power with a mandate to extend the Brexit deadline and then call an election – possibly after a referendum.
No other PM emerges
Once Mr Johnson loses the confidence vote, the opposition parties think they have the numbers to install a replacement.
But they turn out to be mistaken.
Mr Corbyn finds the resistance to his premiership is stronger than he thought, with the LIb Dems refusing to fall into line.
But he in turn refuses to get behind any ‘unity’ candidate, insisting it is his constitutional right to be the next PM.
In these circumstances, a fortnight goes past with Mr Johnson still in No10.
At that point an election is triggered under the Fixed Term Parliaments Act. Parliament is dissolved, and Mr Johnson gets to set the date of the ballot.
Remainers fear in these circumstances Parliament loses control as it is not sitting, and could be vulnerable to any tricks the government tries to pull to avoid delaying Brexit.