Some allies of the prime minister are desperate for a majority of MPs to back Ken Clarke’s motion to keep the UK in the customs union, at the close of round two of the Letwin process of the Commons bossing the government, Monday night.

Yes you heard me right.

They want MPs to vote for a plan that would drive a coach and whole herd of horses through the Tory election manifesto and would cleave the Conservative Party in two.

To be clear, these are not ministers and officials who themselves are keen for the UK to agree a deal with the EU that would remove the requirement for customs checks to be reintroduced after Brexit.

Au contraire.

But they hope if it becomes the revealed will of the Commons to negotiate that kind of so-called soft Brexit - one which would keep the UK under the jurisdiction of the European Court of Justice, and would prohibit the negotiation of trade deals with non-EU countries - that would be the perfect platform for the prime minister to put her own Brexit deal back to the Commons for a record-breaking fourth time - on Tuesday.

The result of Ken Clarke's motion could bring the PM either salvation and damnation. Credit: PA

The PM and her advisers (or at least some of them) assume, not unreasonably, that perhaps another 15 Tory Brexiter MPs could be scared into voting for the EU Withdrawal Agreement if they could see that the conspicuous alternative would be a customs-union-based Brexit-in-name-only.

If only 15 Labour MPs could be converted into supporters too, her deal would scrape though at this spectacularly late juncture - and that might happen if the PM were to write into the body of her latest meaningful-vote motion the essence of the amendment laid last week by the Labour MPs Gareth Snell and Lisa Nandy.

I am reliably told the PM will incorporate the Snell/Nandy plan, which would empower MPs to shape negotiations on the future relationship with the EU, partly because she needs Labour votes and also because the Speaker will only sanction a motion from May that is substantially different from previous ones.

Obviously it would be almost a slam dunk for Theresa May if the DUP’s 10 MPs could cease their opposition to the Withdrawal Agreement - maybe there is still a way for the DUP to aid the PM’s dunking.

But to be clear, unlike Tory Brexiters, DUP MPs won’t be terrified by the prospect of the UK staying in the customs union, because that would in practice turn the backstop they hate from putative trap into the bridge the EU always hoped it would be.

For the DUP staying in the customs union could be associated with finding a way to meet their demand that there should be no new regulatory wedge between Northern Ireland and GB.

The PM may write into the body of her latest meaningful-vote motion the essence of a plan by Labour MPs Lisa Nandy and Gareth Snell, in order to get it through the Commons. Credit: PA

So please don’t laugh but just maybe we are two days away from the PM’s Brexit deal being ratified - which would mean (probably) the UK leaving the EU on May 22 (ministers are confident May 22 would be reinstated by the EU as Brexit day in those circumstances).

And if it doesn’t pass at this fourth attempt, well then we are into mega crisis territory because at that juncture the PM would be mandated by MPs to negotiate with the EU a customs-union-based Brexit.

If she were to agree to what would be heresy for most of her colleagues, Cabinet and the Tory Party would divide and crumble - if she refused, she could well face and lose a no-confidence vote.

Which is why the Mrs May will have very mixed feelings about the Clarke customs-union motion being voted on in just a few hours because it is simultaneously the route to her salvation and her damnation, depending on what happens next.