ConservativeHome’s first law of the Brexit negotiations is that if Theresa May can put off a difficult decision, she will.  It is clear that the options she is mulling include staying in the Customs Union in all but name.  This would suggest an end to her confidence and supply arrangement with the DUP, the loss of the Government’s majority and the resignation of Cabinet Ministers.

So the easiest path to take is the one of least resistance.  Put the moment of decision off.  Let this week’s European Council pass.  Search for compromise as Michel Barnier’s clock to tick on.  This is the form: the Prime Minister postponed a customs decision earlier this year, setting up two Cabinet working groups whose conclusions she then studiously ignored.

But whatever she decides to do, one point is certain.  Geoffrey Cox made it clear to the meeting of senior Cabinet ministers last week that, under the terms of the spring’s draft agreement and last winter’s joint report, no government could unilaterally resile from the backstop.  And it has no end date.  After all, the whole point of the backstop is for it to stand until or unless both sides reach a trade agreement.  This was the issue at stake in David Davis’s pre-Chequers row with May last spring: he was fobbed off with a vague aspiration, not a firm date.

So sooner rather than later – before many weeks are up, even if not this week – May must make her choice.  To go with a customs union and the backstop would have a consequence which many are talking about; and another, no less crucial, which fewer have spotted.  The first is that a UK-wide customs union would shipwreck a central aim of Brexit: agreeing transformational worldwide trade deals.

The second is that Northern Ireland would effectively remain in the Single Market.  So even if the customs union agreement applied to the whole UK, one part of the it would be in the Single Market and the rest would not.  This would break up the Union in much the same way that the Prime Minister claims her Chequers proposal was drafted to avoid.  Let’s think about what would follow.

The DUP sometimes bluffs, like others in politics.  But it surely could not connive in the separation of Northern Ireland from Great Britain.  At this point, May would lose her Commons majority.  Penny Mordaunt, Andrea Leadsom, Esther McVey – all might quit.  So could Dominic Raab, which would be the weightiest blow.  Michael Gove is troubled.  David Mundell would have to weigh the implications for Scotland.  As for Liam Fox, what would be the point of him if his department couldn’t effect trade deals?

Even if all of them clung to their jobs, the ERG would face a moment of truth.  Would Brexiteers be willing to countenance membership of a non-time limited customs union from which future governments could not unilaterally escape?  And would Conservative and Unionist MPs as a whole be willing to tolerate a Prime Minister who praised “our precious union” while at the same time dividing it?

If she lost a leadership ballot, or was so wounded that she had to resign, a candidate would surely stand on the platform that May pays lip service to: that no deal is better than a bad deal.  He or she would argue that the EU sees backstops and concessions and resignations and the entire Article 50 process to date as a sign of weakness.

It would follow that he would put to the EU, say, the David Owen plan: transition followed by continued EEA membership followed by a Canadian-type deal.  He would propose an end-date to any customs union of, for example, December 2021 – the one agreed in outline between the Prime Minister and Davis.  The backstop, together with the rest of the Withdrawal Agreement, would wither on the vine, since “nothing is agreed until everything is agreed”.  And if the EU didn’t agree to these terms, then there would be No Deal.

In short, this person would maintain that there’s only one language the EU understands.  So it’s preoccupied by the fear that Brexit Britain will be free to set its laws, tax and regulation as it pleases?  (Hence the driving reason for its rejection of Chequers in Saltzburg: Barnier presented it to the EU27 as a plot to out-compete them.)  Very well, then: let us make it clear that we’re prepared to do precisely that.

This person sounds a lot like Boris Johnson or perhaps Raab or maybe Mordaunt – though, in sense his or her identity is irrelevant: it’s the strategic plan that matters.  He or she would probably win the leadership election.  But readers will already have spotted the snag.  You can change the leader, but you can’t change the Commons.

There would be no majority for such a No Deal plan.  Dominic Grieve and company have gained a reputation for leaving the kitchen when the heat sizzles up.  But they would surely pile on the pressure for a Brexit postponement and a second referendum.  They would not be alone.  Panic would be up at the prospect of “Britain crashing out”.  The pound would slide; investment would freeze.  (Open Europe’s new report, by the way, suggests the medium to long-term effects of No Deal would be relatively minor: see Stephen Booth on this site today.)

The inevitable consequence, sooner or later, would be an election.  But here’s the problem for Johnson or whoever.  Conservative MPs are petrified of another poll, especially those in marginal seats.  They fear that a red tide will sweep them away.  The more our putative leadership candidate hints at a poll, the lower his vote among his colleagues is likely to be.

We have said since the summer that the key question Tory MPs must ask themselves is: could any deal that May might put to the Commons be worse than the risk of No Deal (if truly unmanageable) or else No Brexit?  Perhaps this is now the wrong question.  Maybe the right one is: is the risk of No Brexit – or a Corbyn Government – worse than a No Trade Deal Brexit that also carves up the UK?

We have also agreed with Gove that, as we like to put it, Brexit is a film, not a photo – in other words, that it will develop over time.  Where we are the day after Brexit is not where we will be after, say, ten years have passed.  But whatever Downing Street may claim to the contrary, treaty commitments are treaty commitments.  They cannot be torn up by one party without severe implications for its international standing – and any future treaties and agreements.

So, to pursue the analogy, Brexit might be a film – but a treaty with the backstop and a no-escape customs union enshrined in it would present us with a Groundhog Day film.  Just as Phil Connors woke each morning in Punxsutawney to the realisation that he’s trapped there, so the UK would face each new day a customs union by which it was constrained, and a backstop by which it was dismembered.

Here, then, is the looming choice for Conservative MPs.  They are trapped between what one classically-minded Cabinet Minister calls the Scylla of the Customs Union and the Charybdis of No Deal.  They are faced with either what is a dire form of Brexit, which risks populist backlashes UK-wide…or the risk of No Brexit at all, or else a No Deal that, in the short-term, has the potential to paralyse the economy.

None the less, the choice is not inevitable.  There is a glimmer of light.  The Prime Minister could drop Chequers, and pursue a Canada-type trade deal, perhaps with Owen’s variant, thus boiling down the negotiating obstacles largely to one – the backstop.  Maybe – just maybe – the EU would then be prepared to slap a unilateral escape clause on it, and so avoid the hard border in Ireland that its present approach, ironically, risks.  If she tries to fudge a decision, why should the Cabinet have confidence in her, and why should anyone else?