This week, the Daily Mail reported that Alan Johnson, the widely-respected MP fronting Labour’s campaign to stay in the EU, had claimed that Welsh independence is a “real threat” in the event of Brexit.

This is not a new tactic. The prospect of Scotland quitting the Union in favour of Brussels has been touted for some time. I expressed scepticism about this on this site back in January, which earned me a ‘Jocksplaining’ from Alex Massie in the Spectator.

Now I’m far from convinced by Massie’s thesis, for several reasons. First, all the commentators who make his case seem to support EU membership anyway. Second, it specifically claims that rational explanation is impossible and that we should take their word for it on account of their being Scottish.

This might be easier if it were not for point number three: that some of Scottish nationalism’s most thoughtful and respected advocates – who are also Scottish – take the opposite view. These include: Alex Bell, the SNP’s former head of policy; Andrew Tickell, who under the pseudonym ‘Lallands Peat Worrier’ is one of nationalism’s pre-eminent bloggers; and Gordon Wilson, a former SNP leader.

But whilst there may thus remain some leeway for we London-based writers to doubt the entrail-readings of pro-Remain Scottish haruspexes, it is nonetheless true that their case is eminently plausible. Scotland does have an record-sized separatist movement, nationalist fervour is greatly distorting her politics, and it is not impossible that she might yet take the ultimate step.

None of these things are true of Wales.

As the Mail points out, not even the Welsh nationalists are calling for an independence referendum in the event that Wales votes Remain but the UK – England! – votes Leave overall.

It isn’t difficult to see why: in the aftermath of Scotland’s 2014 ‘No’ vote the share of the Welsh population supporting independence fell to just three per cent. And the rational case that its harder for Wales to quit the UK outside the EU than in still holds.

Johnson is presumably a sincere believer in British membership of the EU, and he’s no fool. Therefore we must assume he has a range of reality-rooted arguments for Remain in which he sincerely believes. Why would he instead choose to take up so absurd an argument?

My guess would be Carwyn Jones, who claimed in The Independent earlier this week that Brexit could lead to “the Welsh people are asking which union – the UK or the EU – we should be a member of.”

Jones is the First Minister of Wales. He is also, whilst his party is out of power in Holyrood and Westminster and unless and until Khan wins the London mayoralty, the most senior elected Labour politician in the country.

This is unfortunate on several levels. For one thing, Welsh Labour have been in office in 1999 and have used devolution to shield public services from even the softest reforms. This has resulted in a “self-inflicted Welsh education debacle” and an NHS record so weak the Tories could attack on it.

Not a great model for a Labour Party whose primary challenge is making its beliefs and remedies relevant to the challenges of the modern world.

Yet Jones’ (relative) success also makes it much harder for his critics to cut through with their message that, under his stewardship, things are going “very deeply wrong” for Welsh Labour.

By fixating on the ailing Welsh nationalists and constantly striking poses against London, the First Minister has given the Tories the space they needed to rebuild and blind-side him in places like Gower.

Jones is repeating Scottish Labour’s ignoble tradition – and historic mistake – of pandering to anti-UK nationalism for short-term gain, rallying to his standard ministers who dismissed the race to replace Ed Miliband as the “English Labour” leadership contest (despite Welsh members being able to vote).

He does all this seemingly incognisant of the fact that if you turn politics into a “who is most Scottish/Welsh” competition, nationalists will always win in the end.

Labour’s recent mauling at the hands of the SNP has not dissuaded him. Nor has the fact that nationalism in Wales is simply not the force it is in Scotland.

Indeed it sometimes seems as if Jones regrets nothing so much as the fact that Wales has no powerful separatist movement, containable only by endless concessions of more power, prestige and cash to Cardiff Bay. So he tries to pretend it does.

His attempt to conjure the absent spectre of Welsh secession should allow national Labour figures such as Johnson to get a clear-eyed view of their man in Cardiff, who is either a soft nationalist himself or is unabashedly invoking nationalist tropes to undermine British governance and accrue more power to his own administration.

If pro-Brexit unionists have a duty to consider Scotland’s response when making their calculation – as I believe they do – then pro-Remain unionists have a reciprocal obligation not to pick at the ties that bind Britain together in a bid to drum up votes.

There is no moral obligation to devolve foreign policy and no practical risk of Welsh secession. As leader of Labour’s Leave campaign Johnson must find the courage to break with his Welsh colleague and stop peddling this nonsense.

And if he won’t, then another prominent Welsh Remain supporter should tackle Jones head-on. Step forward, Stephen Crabb.