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…

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