British Prime Minister Theresa May. Photo: Paul Ellis/PA Wire/PA Images

Parliamentary votes are often important and sometimes uncertain. However, seldom has a decision been simultaneously so important and so uncertain as the one that MPs look likely to face in a few weeks’ time on the European Union’s customs union.

The government wants the UK to end up with complete freedom to decide how to regulate British businesses and trade with the rest of the world. This means having nothing to do with the customs union. All the opposition parties and a small number of Conservative MPs believe that jobs and prosperity are best protected by negotiating some kind of new customs union agreement with the EU, and amendments to the government’s trade legislation have been proposed with this in mind.

Will the government be defeated and have its whole Brexit strategy thrown off course? As the Americans say, do the math.

The starting point is the government’s overall majority of 13. It enjoys this as long as (a) the ten DUP MPs who signed up to a confidence and supply agreement after last year’s election continue to vote with the Conservatives, and (b) the seven Sinn Fein MPs maintain their refusal to take their seats at Westminster.

This means that if just seven Conservative MPs vote against the party whip, the government will be defeated—if no opposition MPs abstain or vote with the Tories.

The best evidence we have of what might happen in practice is the vote three months ago, on 13^th December, on the amendment tabled by Dominic Grieve, the Conservative MP, to give parliament a “meaningful vote” at the end of the Brexit negotiations. The government lost by four votes, when Grieve was joined by ten other Tory MPs in the division lobbies. (They were: Heidi Allen, Ken Clarke, Jonathan Djanogly, Stephen Hammond, Oliver Heald, Nicky Morgan, Robert Neill, Antiontette Sandbach, Anna Soubry and Sarah Wollaston.) The votes of those 11 rebels were partially offset by two anti-EU Labour MPs, Kate Hoey and Frank Field, who voted with the government.

A good starting assumption is that at least one of the coming votes on the customs union is likely to be something like a repeat of the December vote. However, given that this was so close, it would take very few MPs to produce either a government victory—or a more substantial defeat. So: who might vote differently next time?

– There might be more Tory rebels. Three MPs came close to voting for the Grieve amendment but changed their minds at the last minute under pressure from the party whips: Vicky Ford, George Freeman and Paul Masterton. They could be potential rebels next time. So could two strongly pro-Europeans who have since left the government and are no longer bound by ministerial loyalty in Commons votes: Justine Greening and Damian Green (though Green may be swayed by four decades of personal friendship with Theresa May since they were students at Oxford together.) John Stevenson was a 12^th Tory rebel in December, but cancelled his vote by going through the government lobby as well, so he ended up as a hyperactive abstainer.

– Against that, four strongly anti-EU Labour MPs voted for the Grieve amendment: Dennis Skinner, Grahame Morris, Ronnie Campbell and John Mann. They might feel that it was one thing to abide by the Labour whip on an issue of the rights of parliament—but not on amendments to keep the UK’s trading arrangements close to the EU’s.

– Might Sinn Fein’s seven MPs abandon their historic refusal to take their seats, and take part in the coming votes? They insist they won’t betray their party’s traditions or their promise at last year’s election NOT to take their seats. On the other hand, Sinn Fein also feels strongly about keeping the Irish border open, and a large part of the customs union controversy concerns the future of that border. Voting at Westminster to scupper a policy that might lead to a hard border could boost Sinn Fein’s popularity in the Irish Republic

– Two MPs suspended by the Labour Party abstained in December: Ivan Lewis and Jared O’Mara. They might vote against the government next time, especially if they are admitted back into the party.

My best guess is that voting today would result in government defeat, with small increases in the numbers of both Tory and Labour MPs voting against the party line; though the new Tory rebels are likely to equal or outnumber the new Labour rebels when it comes to the crunch. Sinn Fein is likely to maintain its traditional posture—though if its MPs did take their seats, a government defeat, on these and potentially other Brexit votes, becomes virtually inevitable.

However, the one thing we know is that there will not be any voting on a customs union today. Votes may not be held until May or June. Much could have happened by then to change what happens at Westminster. The array of possible events includes a complete breakdown in the negotiations with Michel Barnier.

Assuming that doesn’t happen, Theresa May might try to make the customs union issue into a vote of confidence, and frighten the Tory rebels into toeing the party line by threatening them with the danger of an early general election and Jeremy Corbyn becoming prime minister. Such a tactic might have worked in the past, but it would be scuppered today by the Fixed-Term Parliament legislation from 2010. This states that a no confidence motion must be precisely that. A majority of MPs must vote for the motion: “That this House has no confidence in Her Majesty’s Government.” Ministers cannot choose to make a vote on anything else a confidence vote. Nor, without looking ridiculous, can they table a no confidence vote themselves: it must, in practice, be tabled by opposition MPs.

That is not to say the government won’t fall. But a defeat on a policy issue, even a major one, cannot in itself precipitate an election, unless enough Tory and/or DUP MPs decide they want one—which, at the moment, they emphatically do not.

Of course, one way to head off a rebellion from the Tory pro-EU rebels would be to concede the need for a long-term, post-Brexit customs union with the EU. But that might provoke a different rebellion: by Jacob Rees-Mogg and the other hardline Brexiteers. The prime minister would then need Labour support, or at least acquiescence in the form of abstention, to keep Brexit on track. For lovers of history, that is how Robert Peel managed to repeal the Corn Laws in 1846. The Tories were split, and he needed Liberal backing to get reform through parliament.

If May is tempted to follow this example, she should note what then happened. Peel himself was forced to resign; and, broken by their internal battles, the Tories spent most of the following three decades in the wilderness.