Boris Johnson has urged MPs to back his newly-won Brexit deal ahead of a knife-edge House of Commons vote this afternoon.

The prime minister, who is facing tense hours of debate before MPs decide whether to approve his proposals, told the Commons a historic Saturday sitting - the first in 37 years - "is the moment we can finally" resolve Brexit.

In an attempt to convince MPs to back his deal, he said it was "the best possible solution" and claimed any further delay to Brexit would be "pointless, expensive and deeply corrosive of public trust".

He said the agreement he has struck with Brussels would allow the UK to leave "whole and entire" on October 31.

But obstacles stand in his way, with opposition MPs threatening to vote for an amendment which could see a further delay to Brexit until all legislation needed to implement the new deal is passed.

If passed, the amendment - tabled by Sir Oliver Letwin - will see the prime minister send a letter to the EU by the end of the day requesting a three-month Brexit extension.

Mr Letwin, a former Cabinet minister who had the Tory whip withdrawn after rebelling over Brexit, said it was an "insurance policy" to prevent Britain "crashing out" without a deal on October 31.

He claimed the purpose of his amendment was to prevent an unintended no-deal exit happening by default if the deal had not been fully ratified by the deadline.

Government sources reportedly warned that if the Letwin Amendment passed, it would render a vote on Mr Johnson's deal "meaningless".

The source said a meaningful vote on the deal would be postponed, if the amendment passed, to allow time for legislation to be introduced which would ratify the new deal.

Labour leader Jeremy Corbyn warned against voting for the prime minister's deal and the party is whipping MPs into backing the Letwin Amendment.

A party spokesperson said MPs should back the amendment to avoid a vote on the Brexit agreement becoming a "blackmail vote between his deal and no deal".

He claimed "voting for a deal today won't end Brexit. It won't deliver certainty and the people should have the final say", adding: "We will not back this sell-out deal."

He told MPs "this Government can't be trusted and these benches will not be duped".

He added: "I totally understand the frustration and the fatigue across the country and in this House.

"But we simply cannot vote for a deal that is even worse than the one this House rejected three times."

Shadow Brexit secretary Sir Keir Starmer warned MPs against backing the deal, claiming it is a "trap door to no deal".

But Mr Johnson called for MPs to reconcile their differences over Brexit.

He told MPs: "The House will need no reminding that this is the second deal and the fourth vote, three-and-a-half years after the nation voted for Brexit.

"And during those years friendships have been strained, families divided and the attention of this House consumed by a single issue that has at times felt incapable of resolution.

"But I hope that this is the moment when we can finally achieve that resolution and reconcile the instincts that compete within us."

The upcoming vote appears to be on a knife edge, with Mr Johnson's one-time allies in the DUP threatening to vote against it.

Its deputy leader Nigel Dodds claimed the deal "drives a coach and horses through the Belfast Agreement" and urged MPs to reject the proposal.

He said: "Weariness in this House over Brexit should not be an excuse for weakness on Brexit or weakness on the union."

Who does Mr Johnson need to convince?

Jeremy Corbyn has stated he is against the deal, claiming the deal Mr Johnson negotiated is worse than Theresa May's previous deal.

In a bid to try and win over Labour MPs, Mr Johnson announced measures to safeguard workers’ rights, including consulting on improving unfair dismissal protections.

But these were savaged by Labour’s shadow secretary of state for employment, Laura Pidcock, who said: “This empty gesture is not worth the paper it’s written on.”

The Liberal Democrats, the SNP and Plaid Cymru MPs have also rejected the deal out-of-hand, leaving Mr Johnson with an uphill task.

He also needs to convince hard-line Tory Eurosceptic MPs in the ERG, who previously voted down Mrs May's bill. The support of Conservative MPs who were expelled from the party in September - most of whom voted for Mrs May's deal - is also seen as crucial.

Will the deal pass through Parliament?

The House of Commons was full for the historic Saturday sitting. Credit: House of Commons

Numbers are tight, it could go either way.

The prime minister doesn't enjoy a majority in the Commons - instead he needs 320 MPs to vote for his deal, assuming all MPs vote and none abstain.

Assuming all Tory MPs vote with the government, it already has 259 votes already in the bag. That means he needs to pick up support from across the House.

The head of the Eurosceptic ERG group has voiced its support for the plan, which would provide a much-needed boost for the prime minister's deal.

If 18 of the 21 Tory rebels who voted for Mrs May's deal but were booted out the party vote for his deal, that brings him to 305 ayes.

The DUP has 10 MPs - but they will almost certainly vote against the deal. That leaves the PM looking for support from Labour rebels to get him over the line.