Boris Johnson has urged MPs to back his bid for an early general election on December 12, with a vote expected around 7pm.
But if they reject his proposal, Mr Johnson will follow a plan "almost identical" to that proposed by the SNP and Lib Dem's for an early election on December 9, a Number 10 source claimed.
And in a surprise admission Labour leader Jeremy Corbyn vowed to "consider carefully" any legislation which "locks in" the date of a general election.
He appeared to offer a warmer response to the earlier national poll mooted by the Liberal Democrats and SNP, rather than the government's bid for a December 12 ballot box showdown.
Labour, with 244 MPs, effectively has a veto against an election in tonight's vote under the Fixed-term Parliaments Act (FTPA), which requires two-thirds of the Commons - 434 MPs - to back one.
During Monday evening's debate the prime minister told MPs that Parliament had "run its course" and should back an election because he said the House was "incapable" of delivering on Brexit or anything else.
He said Labour leader Jeremy Corbyn "can run" from a general election "but he cannot hide forever".
He added: "I hope he accepts tonight that he is snookered, that this charade has gone on for long enough, that he will agree to allow Brexit to get done, and then allow us to make our cases to the people."
He claimed Mr Corbyn had "run out of excuses" for not supporting a general election and claimed it would "make sense" for the Labour leader to back one.
Meanwhile, Mr Johnson wrote to European Council president Donald Tusk to confirm the UK's "formal agreement" to the Brexit extension.
He said the "unwanted prolongation" of Brexit is "damaging to our democracy" but wrote that he'd confirmed "the UK's formal agreement to this extension" to January 31.
Mr Corbyn said the PM's new Brexit deal was a "recycled and rejected deal" that had been "misrepresented by ministers", meaning a no-deal Brexit would still be possible, even if the deal was supported.
"(The PM) said he would never ask for an extension and he said he would rather die in a ditch - another broken promise," he said.
He added: "He says he wants an election on December 12, though how can we trust him, that he will stick to that date when we do not yet have legal confirmation of the extension?"
He urged MPs to reject the call for a general election in favour of taking a no deal Brexit off the table, saying the British public "do not trust" the prime minister.
The DUP - the government's former ally in government - said it would not back the proposals for an election.
And SNP's Westminster leader Ian Blackford said his party would not back the motion for a December 12 election but would support efforts for the earlier December date.
Mr Blackford said: "If we enable this motion to pass, we will be out before the Prime Minister's election.
"We cannot allow the Prime Minister to railroad through this disastrous so-called deal."
He added: "We will support the Liberal Democrats' proposals for an election before Brexit can happen, with no reintroduction of the Withdrawal Agreement Bill.
"Because given the way that some Labour MPs voted we cannot trust Labour to block the Bill in future."
The debate on an election began after EU ambassadors granted the UK's request for a Brexit "flextension" until January 31 2020.
Labour's lack of support for the proposal means it is likely to be defeated when voted upon on Monday evening.
Earlier Labour was dealt both a blow and a boost on Monday, with one of its MPs joining the House of Lords and another facing suspension.
MP Keith Vaz is set to be suspended from the Commons for six months after "expressing willingness to purchase cocaine for another person".
And John Mann left the Labour benches, however, with him voting against the party and for Mr Johnson's deal, it means the PM has lost one MP who supported his Withdrawal Agreement Bill (WAB).
What could happen next?
If the PM's election bid is defeated in the Commons on Monday evening, he may look to secure a December election through other options.
Lib Dem leader Jo Swinson and the SNP's Westminster leader Mr Blackford put forward a tightly-drafted Bill that would grant an election on December 9 - on the condition the European Union grant an extension until January 31.
The SNP/Lib Dem draft law, currently scheduled for Tuesday, would require a simple majority of 320 MPs - 114 MPs less than Monday's vote - to support it in order to dissolve Parliament.
The problem with pursuing this plan, for the government, is that amendments could be added to the Bill, for example that 16 year olds would be allowed to vote in the election.
However now EU ambassadors have agreed to a Brexit extension, the Government is looking introduce its own version of the Bill.
If passed on Tuesday, the Bill is likely to achieve Royal Assent by Thursday and Parliament would be dissolved by the end of the week for the first December poll in almost a century.
Its quick dissolution turnaround period would mean the Withdrawal Agreement Bill (WAB) – the attempt to put Mr Johnson’s Brexit deal with the EU into law – would fail to pass before Halloween.
Another route to Mr Johnson's election could be for him to call a vote of no confidence in himself, which could trigger a general election.
The problem with this route is that under the FTPA the opposition is given 14 days to form an alternative government before an election is called.