In recent days, there has been considerable speculation that a possible "landing zone" for a Brexit deal would be a return to the so-called Northern Ireland-only backstop. This was proposed by the EU — and rejected by the UK — in March 2018, and would have kept Northern Ireland, but not the rest of the UK, in the customs union and the single market for goods. The backstop which Theresa May later signed up to in the withdrawal agreement extended the customs union element of the backstop to Great Britain, too.

Although this is often referred to in Brexit jargon as the "all-UK backstop", this is slightly misleading, as the regulatory aspects of it applied to Northern Ireland alone. The main substantive difference between the two versions of the backstop is therefore the scope of the customs union; the first version being Northern Ireland-only, the second version all-UK.

Once this is grasped, recent speculation that the UK government is considering a return to a Northern Ireland-only backstop looks misplaced. It seems primarily to stem from the Prime Minister’s recent floating of all-Ireland alignment on agricultural and food products.

The EU’s new trade commissioner Phil Hogan, an Irishman, said on Tuesday that Johnson’s proposal “is certainly a clear indication of divergence between Northern Ireland and the Republic of Ireland/the EU and the rest of the UK… the Taoiseach [Leo Varadkar] has indicated in the last 24 hours that the Northern Ireland-only backstop is quite an interesting idea to revisit.”

But North-South alignment on agri-food is a long way from the Northern Ireland-only backstop; in fact, it is already a feature of the current backstop. The key question on GB-NI divergence is not agri-food alignment; it is whether the UK can accept Northern Ireland being in a different customs territory and tariff regime from Great Britain. Indeed, Johnson explicitly ruled this option out yesterday.

Even if Johnson was prepared to present the House of Commons with the Northern Ireland-only backstop next month, there is little chance it would get past MPs. Of course, it seems almost inevitable that the DUP would oppose it, as they did in December 2017, March 2018 and have done ever since. If even the Mayite backstop was too NI-specific for them, there is little reason to think that they, or the wider unionist community, would sign up to an even harder economic border in the Irish Sea. Senior DUP figures have suggested in recent days that they would be open to some NI-specific measures if there was a "Stormont lock" on divergence with Great Britain. But this was in the context of regulatory divergence, not the wider divergence in customs and trade policy that a full-fat NI-specific backstop would entail.

Some have nevertheless argued that Johnson’s expulsion of the 21 Tory rebels means that he has no majority anyway, and can therefore afford to “throw the DUP under a bus” by reverting to the NI-only backstop. However, this rests on the questionable assumption that the government’s previous opposition to the NI-only backstop was purely about the DUP. Not all Conservative MPs are as unionist as they like to claim, but enough of them are to make getting an NI-only backstop through parliament a tall order.

To win a majority, any withdrawal agreement needs to win over at least 29 MPs who voted against it on the third occasion, and keep every MP who previously voted for the deal on board. As far as the Conservatives are concerned, an NI-only backstop fails on both counts. The 28 "Spartan" Tory MPs who voted against the deal three times are unlikely to accept it.

For some, the NI-specific elements in the May deal were a key reason why their opposition was so strong; Bill Cash, John Baron and Owen Paterson all cited this in the debate before the third meaningful vote. Others, such as Mark Francois, Steve Baker and John Redwood, seem unlikely to vote for any withdrawal agreement, regardless of whether the backstop is UK-wide, NI-only, or disappears altogether.

Moreover, an NI-only backstop might also lose the support of Tory MPs who did ultimately vote for the deal, many of whom take their cues on unionism from the DUP. The Scottish Conservatives, for example, will recall that the SNP responded to the original NI-only backstop proposal by arguing that special treatment for Northern Ireland should be extended to Scotland. Indeed, it’s not even clear that May herself would vote for an Irish Sea customs border, which she denounced so many times as anathema to unionism.

It’s also worth remembering that dozens of Conservative MPs, mostly Brexiteers, voted against amendments from Labour MPs Stella Creasy and Connor McGinn on abortion and equal marriage earlier in the summer. A quick scan through the Hansard debates shows that most of these MPs cited the devolution settlement, not social conservatism, as their motive for doing so. This reinforces the point that a considerable number of Tory Brexiteers are aligned to the DUP on constitutional arguments such as Northern Ireland’s place in the Union. For that reason, it’s not even clear that a Tory majority government, unencumbered by the DUP, would plump for a Northern Ireland-only backstop, rather than the seductive simplicity of a no-deal exit.

The parliamentary problems with the Northern Ireland-only backstop don’t stop there. Yes, the caucus of Labour MPs who say they will vote for a deal has reasserted itself in recent weeks, including some who voted three times against the existing deal. But most of the "Labour for a deal" group want a relatively soft Brexit — ideally a customs union, as much regulatory alignment as possible without free movement of people, and protections for workers’ rights.

In contrast, the whole point of a Northern Ireland-only backstop is that it would give Great Britain the freedom to pursue a hard, Canada-style Brexit — the exact opposite of what Labour MPs want. If pro-deal Labour MPs were to vote for that (judging it, perhaps, as preferable to no deal or no Brexit), then it would highlight the strategic error they made in rejecting the May deal, which is far closer to what they say they want.

In short, it’s not clear that a Northern Ireland-only backstop suits any of the swing votes in parliament needed to pass a deal. There may be a handful of Conservatives who are a) willing to sacrifice their opposition to an Irish Sea border for the sake of a buccaneering, global trade policy for Great Britain; and b) willing to accept the rest of the withdrawal agreement (the transition period, financial settlement, et cetera).

But for many other MPs, Conservative, DUP and Labour alike, a Northern Ireland-only backstop would not just be a bad deal, but worse than the deal that many of them have already rejected. Speculation about a Northern Ireland-only backstop may ultimately be little more than wishful thinking on the part of the EU. There may yet be a landing zone for a deal, but this is not likely to be it.

Dominic Walsh is a policy analyst at the think-tank Open Europe. He tweets @DomWalsh13