“As we leave the European Union, we will no longer be members of the Single Market or the Customs Union but we will seek a deep and special partnership including a free trade and customs agreement.”  With these words, last year’s Conservative Manifesto ruled out membership of any union on customs – in other words, any deal that leaves Britain’s trade policy as “part of the Common Commercial Policy and being bound by the Common External Tariff”.  These were the elements of the Customs Union that May specifically rejected in her Lancaster House speech last year.  Being free of them is the difference between a union and an agreement.

Theresa May would thus be entitled to make any significant vote on the matter one of confidence – as some senior Ministers are today reported to believe she should.  There are three arguments against doing so: one footling; one intriguing. And one that is unmissable – whatever one’s view of it.

The first is that the Prime Minister can’t declare a vote to be one of confidence because of the requirements of the Fixed Terms Parliament Act.  This claim holds in principle, but dissolves in practice.  If May tables a motion calling for a general election, Jeremy Corbyn won’t stand in her way.  We know so because, last summer, that’s what happened.  The Prime Minister won such a vote by 522 votes to 13.  That was the two-thirds majority needed, and a lot more.

The second is that any such vote would necessarily be on an amendment to a Government Bill (or more than one).  May is surely not going to seek a election over a vote that is merely declaratory – no matter how damaging such a development might be to Britain’s negotiating position.  No, the vote of confidence that those senior Ministers favour would be related to specific legislative amendments.    And legislation, like oaths in A Man for All Seasons, is “made of words”.

As John Rentoul points out over at the Independent on Sunday, the wording of at least one of such amendments requires careful scrutiny.  An amendment tabled by Anna Soubry to the Trade Bill would if passed instruct the Government to “implement an international trade agreement which enables the UK to participate after exit day in a customs union with the EU”.  Readers will note that it does not seek to keep Britain in the Customs Union.

This is doubtless because we cannot stay in it, strictly speaking, without also staying in the EU – and thus breaking faith with the referendum verdict.  Soubry, who voted for Article 50, will know this very well.  Hence the wording – a customs union.  Furthermore, the amendment seeks to “enable” the UK to participate in such a union.  One might buy a car that enables one to drive, say, to Broxtowe.  But one might not choose to do so, even if enabled to.

None the less, such a Customs Union provision is not one that could safely be left in the hands of Jeremy Corbyn.  And there may be other amendments with wording more inconsistent with May’s manifesto commitment.  Which brings one to the third argument against making a vote to put Britain in a customs union one of confidence – namely, that the Government might lose it, plunge the country into an election it doesn’t want, and risk Britain’s Brendas from Bristol returning Corbyn in a spasm of exasperation.

Any Conservative MP who voted against a Tory Government on a vote of confidence would be bound to lose the whip – whether such a vote was successful or not.  It is a step that the Maastricht rebels, that emblem of backbench defiance, were not prepared to take.  It might be that some pro-Customs Union Conservatives who have made up their minds not to stand in any snap election might be prepared to act otherwise.  (Our questioning gaze is drawn, naturally but sorrowfully, towards Ken Clarke.)

But the prospect of breaking faith with one’s Association and colleagues, not to mention cutting short one’s career and pension, will not  be an appetising one to pro-Customs Union Tories.  After all, Douglas Carswell left the Conservatives voluntarily in a blaze of publicity.  And he had what was then a lively political party behind him. Neither condition would apply to any Tory MPs prepared to break faith with the manifesto over the Customs Union.  Like him, they would face being out of the Commons – probably faster.

All in all, there are three possibilites if a vote on a Customs Union is declared one of confidence.  First, some Tory MPs vote with Corbyn, a general election follows – and, whatever the result, they are remembered as the Conservatives who were willing to risk putting a Marxist into Downing Street.

Second, some vote with Corbyn…but an election doesn’t follow, because the Government wins the vote.  That would leave May another x votes down, after the whip was withdrawn from those rebels.  She has already lost Charlie Elphicke.  She can’t afford to lose any more.  Finally, a no confidence gambit works.  This is the most likely outcome, and May must prepare for it.  She should not shirk from declaring any vote on a Bill one of confidence if necessary.

However, we are not there yet.  One of the reasons the Government has postponed the Trade Bill is that it wants time to hammer out a compromise – though one that will keep Britain off the Common External Tarrif.  The Prime Minister’s speech this coming Friday may offer some clues.  But whether it does or not, Ministers must keep talking a lot, listening even more – and, above all, put the words of any important amendment under a magnifying glass.  “What do you read, my Lord?”  “Words, words, words.”