The Northern Ireland backstop as presently drafted would, were Olly Robbins’ Brexit plan were to be implemented, apply a customs union to the whole United Kingdom. This would actually represent a diplomatic win for the Government, because the EU’s previous position was that the backstop must apply only to Northern Ireland, according to its reading of Articles 49 and 50 of last year’s outline Withdrawal Agreement. (Let us not be detained by the disaster that permanent customs union membership would suggest for our freedom to strike meaningful global trade deals, a Brexit essential.)

However, the Robbins proposal would only solve half of Northern Ireland’s problem, from a pro-Union point of view. This is because while the province would get no special treatement in relation to a Customs Union, it would get this in relation to the Single Market. In effect, Northern Ireland would stay in it. This would harden and deepen the regulatory barrier that already exists in the Irish Sea.

It is all a question of degree. As we say, there are already some checks, mainly on live animals and other agricultural products. The island of Ireland is already a single epidemiological area. Our sense is that Conservative Brexiteers and the DUP are ultimately willing to sign up to continued full regulatory alignment in this respect. There is also an all-island of Ireland single electricity market. But a huge uptick in east-west agricultural checks and Single Market absorption altogether would be a giant step too far. Northern Ireland would be in the Single Market and the rest of the UK would not. That would surely be too much for the DUP. Its support for the Government would collapse and the Prime Minister would be left without a majority.

The Robbins solution would also, frankly, make a liar of her. Her precise objection to the EU’s version of a Brexit Canada-type settlement is that it would draw exactly such a higher regulatory barrier in the Irish Sea. Check out her recent “Road to Brexit” video where she says that the present EU version would keep Northern Ireland in “…parts of the Single Market. That would would break up the UK economically, and creating new barriers to our own internal market”.

But the regulatory implications of the Robbins plan are being less thoroughly probed than the customs ones – especially when it comes to Scotland. David Mundell and Ruth Davidson have written jointly to Theresa May to say that “we could not support any deal that creates a border of any kind in the Irish Sea and undermines the Union or leads to Northern Ireland having a different relationship with the EU than the rest of the UK, beyond what currently exists.” The letter has duly found its way into the public domain, together with briefing that both would resign were the Government to sign up to the Robbins solution.

The SNP is trying to portray the letter as signalling support by the pair for a “hard Brexit”. This gambit deliberately misses the point. Davidson and Mundell will not be motivated by support for a clean Brexit, in which the UK leaves the Single Market. Indeed, Davidson is on record as supporting continued membership.

Rather, both grasp that the Unionist case in Scotland would be severely damaged were Northern Ireland to remain in the Single Market while Scotland leaves it. Nicola Sturgeon would claim that the Government was deliberately disadvantaging Scotland. Given the pro-EU sentiment there, the claim would carry electoral weight. The Robbins plan would thus put rocket boosters under Scottish independence. And up with that Mundell and Davidson presumably would not put. The former is the Party’s most enduring Parliamentary representative with a Scottish seat. His loss would be serious.

Davidson’s would be in another dimension altogether. She is the great enduring hope of the Party’s centre-left, for all her recent ruling-out of eventually coming south and seeking the Conservative premiership. Andrea Leadsom, Penny Mordaunt, Esther Mcvey, Liam Fox, Michael Gove, Dominic Raab: all these Ministers backed Brexit.

So for any of them to resign would be one thing – very serious, of course, and the possible spark to a leadership vote. But for Davidson to go would be another. Were she do so, May would stand alone, bereft of backing from her Remainers as well as Leavers, from her Left as well as her Right – from the very heart of the modernisation project. This morning, she seems to backing off the Robbins solution. But we repeat: it isn’t just pro-Brexit MPs who should be watching the Prime Minister carefully. It’s pro-Union ones – in other words, all of them.