So what happens next?

The Daily Telegraph's Europe editor, Peter Foster, has outlined some of the possibilities in the event of Parliament either voting for or against Theresa May's withdrawal agreement.

He concludes that, one by one, "the branches of the decision tree are being pruned away".

Alongside the protesters outside Parliament, a bookmaker was on hand to display the odds for the runners and riders hoping to succeed Theresa May as prime minister.

Mrs May previously said she would step down if her deal was passed by the Commons - an outcome that is far from certain.

Image source, Getty Images

You can read more on this story here.

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So tomorrow MPs will vote on whether to endorse the withdrawal agreement the UK negotiated with the EU back in November - but what is it?

Well...it's the 599 page legal document that sets out how the UK will leave the EU.

It authorises a transition period until the end of 2020, during which the UK will need to abide by all EU rules, but will lose membership of its institutions.

It also sets out details of the money the UK will pay to settle its legal obligations, and of a new UK-EU "joint committee" that will police the deal and any disputes.

The backstop plan for avoiding a hard Irish border is contained in an annex.

Read more.

Commons Speaker John Bercow and Labour MP Chris Bryant clashed in the Commons earlier over how long each other was taking to make a point.

Later, Mr Bryant insisted on Twitter: "I think we were both smiling."

Laura Kuenssberg
BBC political editor

The BBC's political editor, Laura Kuenssberg, told the BBC News Channel that at the moment it looked unlikely that Theresa May's deal would pass, but by tomorrow afternoon the picture might look "very different".

She said: "As things stand, Labour is not going to come on board, although some of their MPs might back Theresa May tomorrow.

And the DUP, her vital Northern Irish allies, are not in the mood to play ball either.

So at best the numbers look extremely tight for the prime minister.

But you know this is all part of Number 10's - maybe strategy sounds too grand a word - it's like their day-by-day effort, right now, to find another scrap, another chance, another step, another way forward, never knowing if they will be able to succeed but determined to keep going.

But of course, that means at the same time Parliament is taking more and more control of this situation, trying to find a compromise that could work for different parties. Although not everybody would be happy with whatever they're able to come up with."

And a reminder about the petition calling for the government to revoke Article 50, which would keep the UK in the EU.

The petition is still due to be debated on Monday at 16:30 alongside two other Brexit-related petitions. 5.9 million people have so far signed the Article 50 petition.

Read more about the petition here.

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You could be forgiven for being confused about what exactly MPs are voting on tomorrow and how we got to this point.

Here we explain the latest vote the government has given MPs and what it could mean for Brexit.

Labour's shadow Brexit secretary, Keir Starmer, has posted several tweets citing "four key reasons" why, he says, it is not possible to separate the withdrawal agreement from the political declaration.

Tomorrow MPs will be asked to vote on the Brexit withdrawal agreement, which covers the "divorce bill", citizens' rights and the controversial Irish "backstop".

In the two previous votes on the deal that the government lost, the two came as a package.

When asked whether Labour will support the government motion tomorrow, the party's leader, Jeremy Corbyn, said "No".

"What the prime minister has tried to do is do something that she denied she would do on 14 January – and that is separate the withdrawal agreement from the political declaration, from the future arrangements.

“Well you cannot separate them, because otherwise you move into a blindfold Brexit on the basis of the withdrawal agreement.

“There is no way out of it (the withdrawal agreement) once you’ve signed it and gone into it, and we are not prepared to support the prime minister on this.

“We want to see a discussion about both of the issues.”

Vicki Young
Chief Political Correspondent

The primary thinking behind the government's decision to ask MPs to consider a motion on just the EU withdrawal agreement, is to secure the extension to Brexit until 22 May.

The BBC's chief political correspondent Vicki Young says: "The EU has made it very clear that if the withdrawal agreement can go through by 11 O'Clock tomorrow night then we get that automatically...

"What this isn't is the so-called 'meaningful vote three'. There have been two votes already and Theresa May has been defeated on those...

"The question that is still slightly unresolved is what does the government have to do to fulfill the UK law which says you have to pass the meaningful vote - which takes in both parts - the withdrawal agreement and the political declaration that talks about the future relationship.

"And the big question is still, do they have the numbers to get this through tomorrow?"

Shadow Brexit secretary Keir Starmer has tweeted, external that Labour will not support the government in Friday's vote as it is "not in the national interest".

He outlined four "key reasons" as to why the withdrawal agreement could not be separated from the political declaration - including the PM saying the two documents should be treated as one.