The real question tonight is what the Prime Minister wants to do after losing and whether or not she will be able to get her way.
There is an almost unlimited number of possible scenarios of what might happen in the aftermath of a defeat and much will depend on the margin in tonight’s vote.
Mrs May will speak to MPs after the vote tonight to give her immediate response. And whatever happens, she will have to set out her plans before Monday as a defeat in the Commons last week requires her to table a plan B motion within three sitting days.
Here are some of the ways the next stage of the Brexit drama might play out:
Theresa May (pictured in Downing Street today) will speak to MPs after the vote tonight to give her immediate response. And whatever happens, she will have to set out her plans before Monday
1. May renegotiates with Brussels and wins a second meaningful vote
This appears to be the Government’s current strategy. After MPs vote down the deal, Mrs May will fly to Brussels to ask for help.
She either gets some new concessions or the EU says there really is nothing else. The PM is expected to ask again for a hard deadline on the lifetime of the backstop.
In the meantime, Mrs May wins a vote of confidence in the House of Commons in an embarrassing failure for Jeremy Corbyn.
While the negotiations continue, it is expected that the stock and financial markets will fall – with the losses likely to build the longer it takes.
Either way, Mrs May comes back to the Commons and asks MPs to vote again – and wins as rebels back down in the face of no deal chaos and plunging markets.
2. May renegotiates with Brussels but loses a second vote – triggering a confidence vote and a new Tory PM who orders a second referendum
Mrs May’s deal is defeated a second time in the Commons and Labour immediately calls another vote of confidence in the Government – which this time is lost as Tory remain rebels fight to avoid a no deal Brexit.
Mrs May resigns as Prime Minister and is replaced in a quickfire Tory leadership contest. The winner is installed in No 10 but warned by the DUP the deal is still unacceptable.
Admitting there is no way to win a confidence vote and no renegotiation with Brussels, the new PM orders a second referendum on contesting her deal or Brexit with no deal at all.
This would require whoever is Prime Minister to delay Brexit by extending the Article 50 process.
3. May’s Brexit deal is rejected a second time in the House of Commons – she resigns and the new Prime Minister calls for a General Election to break the impasse
Mrs May resigns immediately after her deal is rejected by the House of Commons for a second time. She stays on long enough in No 10 for a Tory leadership contest.
The new Prime Minister declares they want a mandate for their own version of Brexit and says they want a majority in the House of Commons to deliver it.
Labour seizes on the chance to go to the polls and Tory MPs reluctantly vote in the Commons for a snap election in late February.
The Government is re-elected with a majority and passes its version of the deal – but delays Brexit day until mid July to pass the necessary laws.
4. May switches to support for a No Deal Brexit after MPs reject her plan – but Labour wins a vote demanding a new referendum with support from Tory Remain rebels
After Mrs May’s deal is rejected by MPs, the Prime Minister insists the will of the Commons is clear and Brexit must be pursued without a deal.
She wins a confidence vote among all MPs and is able to cling to power while she drives the country toward exit day without a deal.
Labour demands an election but ignored by the PM, takes its earliest opportunity to table a motion in the Commons which condemns a no deal Brexit and calls for a new referendum on Brexit.
The motion passes – while it has no effect in law it changes the political mood dramatically. The Prime Minister says she cannot ignore the clear will of Parliament and starts the process of a new referendum.
Labour leader Jeremy Corbyn (pictured leaving home today) is thought likely to call for a vote of no confidence in the Government as soon as tonight – opening the door to the PM being ousted
5. May is replaced by a new Tory Prime Minister who immediately calls an election – but Labour wins with a promise to renegotiate
After losing her deal in the Commons, Mrs May is replaced in a rapid leadership election. The new Tory Prime Minister immediately calls an election.
Labour campaigns on its promise to renegotiate the deal. Polling day comes in late February and Mr Corbyn is returned as Prime Minister with a small majority.
He immediately goes to Brussels, gets a three month extension to Article 50 and restarts talks for Labour’s plan of a permanent customs union that gives Britain a special say over EU trade deals.
6. The deal is defeated and after a no confidence vote, Remain MPs break party lines to back a new PM to form a national government that calls a referendum
Mrs May resigns amid chaos following a confidence vote – starting two weeks of limbo before a new election must be called in the absence of a Government.
In the vacuum, an MP – perhaps ex Tory minister Nick Boles – calls on people across the Commons to break party lines and form a national government.
The group forces a motion onto the floor of the House, possibly via the backbench business committee.
In a surprise result, the motion not only carries but has support of more than half of MPs – shifting the politics and prompting the new leader to be invited to form their national government.
The new Prime Minister calls a referendum with Remain, No Deal or May’s deal on the ballot paper – with a second round run off to decide the final settlement.
How will MPs make their decision on May’s Brexit deal and what will they vote on?
MPs will finally give their verdict on the Brexit deal tomorrow night – but how do they vote and what will they vote on?
What will MPs vote on?
MPs will be asked to approve or reject Theresa May’s Brexit deal.
The deal is contained in two documents, the Withdrawal Agreement, which is a treaty, and the Future Framework, which is a political statement agreed between the UK and EU.
But before the main vote, MPs will be asked to say yes or no to a raft of amendments. These are re-writes of the main Brexit deal motion that have been tabled by backbench MPs and Labour leader Jeremy Corbyn.
How many amendments will be voted on?
Commons Speaker John Bercow has selected four amendments from more than a dozen on the order paper.
What do the main amendments say?
Amendment A: Labour leader Jeremy Corbyn’s amendment rejects the deal and says no deal is also unacceptable. It says the deal should include a permanent UK-EU customs union, alongside protections for workers’ rights and the environment.
Amendment K: SNP leader Ian Blackford’s amendment rejects the deal and calls for Article 50 to be extended to avoid a no deal Brexit and notes that the UK could choose to cancel Brexit altogether.
Amendment B: Tory MP Sir Edward Leigh’s amendment would approve the deal but warn the EU making the backstop permanent would breach the Vienna Convention and obliges the UK to scrap the divorce treaty in 2022 if the backstop is still in force.
Amendment F: Tory MP John Baron’s amendment approves the deal but only subject to the EU agreeing to give the UK a unilateral way of ending the Irish border backstop.
When will the vote happen?
Votes will begin at 7pm tonight, after the Prime Minister makes the final speech of the five day debate.
Each ‘division’ of the Commons takes around 15 minutes as MPs have to physically walk through the aye and no lobbies to cast their vote.
This means the main vote will happen at around 8pm and the final score will be known around 8.15pm.
Which order will the votes happen?
The first vote will be on Mr Corbyn’s amendment, followed by Mr Blackford, Sir Edward’s and then Mr Baron’s.
The final vote will be on the main motion – with whatever amendments have been agreed attached to it.
Can voting end early?
If any of the amendments are accepted, those following it automatically fail. There will be then be a final vote on the motion as amended – but this could be nodded through by MPs.