This morning’s papers are full of stories about whether or not the Prime Minister can cobble together a Commons majority for his latest proposals.

But despite reports that both a number of the ‘Whipless 21’ and a significant number of Labour MPs, might be prepared to walk through the lobby with Johnson and his Democratic Unionist allies, it is not yet at all clear that the EU will accept the plan.

Or, indeed, that they’re even meant to. It is not implausible to suggest that Boris Johnson’s plan, which involves both establishing a customs border on the island of Ireland and giving unionists a veto on breaking from alignment with the mainland, is intended more as a bid to make the Government appear the reasonable party in the event of a no-deal exit – although Tom McTague suggests this is not the case:

Even if they’re sincere, the path to a deal is fraught. As Greg Hands hinted at in ConservativeHome’s conference fringe on his Alternative Arrangements Commission, London and Dublin are not really trying to find different technical solutions to the same end-point. There is a political misalignment between what each side considers an acceptable level of post-Brexit continuity, and absent the threat of a no-deal exit the Irish Government has little motivation to, as McTague puts it, “step down from perfection.

Which is not to say that these proposals would not, if accepted, represent a serious concession – perhaps even the first of many – even if Ulster would theoretically re-align with Great Britain in 2026. One commentator has suggested that Britain becomes “more federal”, but that isn’t really accurate when he admits it likely involves an “enhanced” role for Ireland in the governance of British territory.

Owen Polley, who used to work for the Northern Irish Conservatives, sets out the problem in CapX:

“If Northern Ireland is under the political and economic control of the EU until 2026, while the rest of Britain forges an independent trade policy, that situation will become the status quo. It will have practical consequences that weaken the Union and the eventual political convulsion required to reassert British interests in Ulster will be more traumatic.”

Concluding, he adds:

“Even if the Government’s ‘final offer’ is designed to provoke Brussels into issuing a rejection and bringing about ‘no deal’, as Boris Johnson’s critics allege, it compromises the important principle that Northern Ireland should have the same relationship with the EU as the rest of the UK, after Brexit.”

This last is particularly important because, although it seems to have been at least temporarily forgotten by all involved, there was once another reason why unionists were opposed to the backstop. Notwithstanding the specific case of Northern Ireland, they worried that it might set a precedent which would allow other separatist parties, most obviously the SNP, to demand special treatment in turn.

Readers may remember that this ended in a badly-justified u-turn by Ruth Davidson and David Mundell, with Adam Tomkins sent out to try to explain why differential treatment for Ulster had suddenly ceased to be a threat to the integrity of the United Kingdom.

It is worth remembering, if three years of wrangling over the Belfast Agreement wasn’t reminder enough, that there is a huge, qualitative difference between any level of devolution delivered inside the UK’s internal constitutional settlement and baking divergence into an international treaty. Much like Theresa May’s lamentable capitulation over “post-Brexit devolved powers”, Johnson risks escaping a tactical difficulty only by conceding and setting in law principles which undermine the integrity and even legitimacy of the United Kingdom as a nation-state.

Both have accepted, intentionally or not, a position which posits that the UK is less entitled to institutional integrity and coherence than the European Union – Johnson in the manner set out by Polley, and May by legitimising the idea that market-coordination powers may be legitimately pooled in Brussels but not in London.

So far the SNP using the backstop to demand a similar deal for Scotland has been the dog which hasn’t barked. But even if it doesn’t, the Prime Minister must be more careful about laying time-bombs beneath the foundations of the Union. He will already have to devote considerable energy post-Brexit to defusing those bequeathed by his predecessor.