How MPs could vote on the Brexit deal

A total of 635 votes will be in play when the deal is debated.

This means the Government will need at least 318 votes to be certain of a majority.

The 635 votes in play are the 650 MPs in the House of Commons minus seven Sinn Fein MPs (who do not take their seats), the Speaker and three deputy speakers (who do not vote), and the four vote tellers – two Conservative MPs in favour, two Labour MPs against – whose votes are not included in the overall result.

Here are four scenarios for how the vote could unfold.

– Scenario one

If every Conservative MP who is able to vote also backs the deal, this gives the Government 285 votes – 33 votes short of a majority.

This shortfall could be met through a combination of votes from Labour and Independent MPs.

For example, if 12 Labour MPs decided to back the deal along with 23 Independents, this would be enough to see Boris Johnson over the line.

There were five Labour MPs who voted on March 29 for Theresa May’s version of the Withdrawal Agreement: Kevin Barron, Rosie Cooper, Jim Fitzpatrick, Caroline Flint and John Mann.

It is not clear how all of them intend to vote on Boris Johnson’s deal, though John Mann has said he will back it.

A small handful of other Labour MPs have indicated they might consider backing Mr Johnson’s deal, such as Ronnie Campbell and Graham Stringer, though again the precise number is uncertain.

Among the Independents, two former Labour MPs backed Mrs May’s deal – Ian Austin and Frank Field – along with the former Lib Dem MP Stephen Lloyd and the Independent MP Sylvia Hermon.

Other Independents likely to back Boris Johnson’s deal are the former Tory MPs Charlie Elphicke and Nick Boles.

Then there is the group of former Tory MPs who sit on the Government benches as Independents.

There are 21 of them in total – 20 who lost the Tory whip when voting for the Benn Act (to rule out no-deal), plus Amber Rudd, who left the party in protest at Boris Johnson’s Brexit policy.

If most of these Independents back Mr Johnson’s deal, then together with other Independents and a few Labour MPs, the Government might well reach the magic number of 318.

– Scenario two

Not every Conservative MP may decide to back the Government.

A handful of Tory MPs might follow the example of the DUP and vote against.

If 10 of them do so, this would reduce the number of Tories in favour to 275, leaving the Government 43 votes short of a majority.

But it could be the case that more Labour MPs than expected support Boris Johnson’s deal.

Were 20 Labour MPs to vote in favour, plus 23 Independents, then this – plus the 275 Tory MPs – would be enough to get the Government to 318.

– Scenario three

If Labour leader Jeremy Corbyn manages to keep the number of Labour MPs backing the Government to – at most – a dozen, and a handful of Conservatives vote against the deal, then Boris Johnson could be in trouble.

In this scenario, 275 Tories vote in favour, along with 23 Independents but only 12 Labour MPs. This would leave the Government with 310 votes, eight short of a majority.

– Scenario four

MPs might vote in favour of an amendment that requires the Government to ask for an extension if other Brexit-related legislation has not been passed before October 31.

This could mean that the deal is approved in principle on Saturday, but the UK would not leave the EU until the full Withdrawal Agreement – which implements the deal – is also passed.

The amendment has been submitted by Sir Oliver Letwin, one of the group of former Conservative MPs who lost the Tory whip when voting for the Benn Act.

It is supported by others in the group, including David Gauke, Philip Hammond and Dominic Grieve.

If this amendment becomes part of the Government motion on Saturday, then a simple yes/no vote on the overall motion could turn into a choice between “yes by October 31” and “yes but with more time”.

Assuming the Government backs “yes by October 31”, this could prompt many of those Independent MPs to back “yes but with more time” and leave the Government short of a majority.