Jeremy Corbyn caused much consternation on both sides of the Irish Sea when he attacked the Irish backstop after meeting Theresa May for Brexit talks last week.

The point of that summit had been for both leaders to explore potential grounds for a cross-party compromise – or at least to pretend to. Labour’s big ask was a permanent customs union, but Corbyn’s strident criticism of the lack of a unilateral exit clause in the backstop was the dominant theme of subsequent broadcast interviews.

Not only did this upset Dublin, Labour MPs and Northern Irish politicians, but it completely blindsided relevant ministers in the shadow cabinet, who thought the issue had been settled.

With the DUP’s support for the government secured for the medium term, the opportunity to use the issue for narrow political advantage at Westminster has passed. Labour’s official line, first deployed by Keir Starmer last month, is more Peter O’Hanraha-hanrahan than Ian Paisley: they don’t like the backstop, but they’ll have to go along with it.

But Corbyn’s broadside threatened to derail that attempt to finesse a difficult but inevitable transition, from insisting Labour could do a deal without a backstop to voting or advocating for a withdrawal agreement that would still contain one. One shadow cabinet source admits to having no clue as to why Corbyn took such a belligerent tack. “We thought we had got him off that line. He messed up,” they say.

While the Labour leader does have ideological gripes with the backstop – which, among other things, includes alignment on EU state aid rules – some fear that in voicing them he is wasting time fighting the last war. That is why there is a concerted effort afoot to change Labour’s tune on the issue. Starmer and Tony Lloyd, Corbyn’s Northern Ireland spokesman, made a flying visit to Belfast yesterday, where they sought to reassure businesses that, unlike the prime minister, Labour is not disputing the need for a backstop.

Whether the line will hold depends on whether Corbyn, who struggles to say things that he does not believe, can reliably repeat it in public. Recent events suggest that might well be difficult. It’s easy to dismiss as small beer, given the inevitability that any deal will contain a backstop. But the difficulty Labour has had reaching and maintaining internal consensus on the backstop suggests navigating the bigger decisions that will soon come will be even more fraught for the leadership.