Will Theresa May sink or swim in her customs union swimming pool?

On the controversial - some would say "life or death" - question of how to keep open the border between Northern Ireland and the Republic of Ireland, the backstop, this is what I am told has been agreed.

It is what's described in Brussels as the "swimming pool" approach - in other words it has a shallow end and a deep end, when it comes to measures aimed at making sure trade is completely frictionless between NI and the ROI, and fairly frictionless between Great Britain and the EU27.

GB would be in the shallow end, NI in the deep.

Or to be more precise, the whole of the UK would stay in the customs union if a long-term trading relationship between the UK and EU isn't negotiated and implemented by the end of 2020 - which no one (with the possible exception of the PM) expects it to be.

In fact, most EU politicians expect the backstop to be the reality of our trading relationship with the EU for many years.

But - and this is reassuring for May, and perhaps for most in the Cabinet - EU leaders don't really like this version of the backstop, which is largely May's preferred model of how to keep open that border in Ireland (in that sense it can be seen as a victory for her in the negotiations).

So there will be an option after the UK leaves the EU next March, during the 21-month transition period, to negotiate some other arrangement that would have the same effect as the backstop (and yes I know this is confusing - but probably what mostly matters is that only the backstop will be legally binding, and that is what will upset many of her MPs).

The Irish border has proved a constant sticking point. Credit: PA

But in addition to being in the customs union, Northern Ireland would also remain in much of the single market, that part of it pertaining to goods: Northern Ireland alone would be forced to follow all EU directive and laws in relation to goods flowing back and forth between NI and the ROI.

Now that will infuriate the DUP, whose 10 MPs sustain Mrs May and the Conservative Party, because they will see it as doing what they say the PM promised never to do, namely introduce a new border between GB and NI in the middle of the Irish Sea; it would, they fear, increase the danger of a fragmentation of the nations of the UK.

The EU will work hard to reassure Northern Ireland's unionist politicians that there would not be a border in the Irish Sea in any meaningful sense.

But this aspect of the backstop - the deep end for Northern Ireland - could well undermine the agreement between the DUP and the Conservative Party for the DUP to back all important government legislation, and possibly wreck it, throwing into jeopardy the PM's ability to govern.

And if the DUP votes with Labour and hardcore Tory Brexiters AND hardcore Tory Remainers against her Brexit deal - which is an anti-May coalition I anticipate - the PM would lose the "meaningful vote" on the deal by a large margin.

The point is that many of May's Brexiter MPs will hate that GB even in the shallow end will be forced to follow EU rules on competition, the environment, goods standards and employee protection - because the EU is insisting that if the UK derives the benefit of being in the customs union, it has to compete on a "level playing" field. And for as long as the UK is in that customs union, it will be prohibited from doing trade deals with non EU countries.

Cabinet will get a glimpse at the Brexit deal on Wednesday. Credit: PA

Many Remainers share the Brexiters' concern that May's version of Brexit would see the UK as a relatively powerless taker of EU rules.

As I write, I do not know whether the PM will get this proposal through her Cabinet. But even if she does, I have literally no idea how she would get it through Parliament.

For the avoidance of doubt, what's finally been negotiated is much less toxic to May than the EU's original proposal of Northern Ireland alone remaining in the customs union and the single market. But it will still upset many of the MPs who sustain her in power.

To state the obvious, much will hinge on how and whether the UK could get out of the backstop - about which we will presumably learn all on Wednesday.