Christine Shawcroft, a one of Momentum’s directors and a member of Labour’s ruling national executive committee, has resigned her post after the Times obtained a copy of an email in which she called for Alan Bull, a council candidate in Peterborough who has been suspended by the Labour Party after he posted an article on Facebook that called the Holocaust a “hoax”, to be readmitted to the Labour party. (Bull claims that the posts had been taken out of context and that other posts by him had been doctored to look more offensive than they were.)

Shawcroft’s tenure on the disputes panel was already set to be shortlived, as she had voluntarily stepped down from the Momentum slate in advance of fresh elections to the member’s division of the NEC in June. Ordinary Labour members elect nine representatives to the 39-strong body, with the main battle between the official Momentum-backed set of left candidates against the Progress-Labour First backed roster of centre-left candidates.

It is widely believed that the reason she and Rhea Wolfson opted not to stand again is because members of the Momentum slate are required to sign up to a guarantee that they will not seek selection to parliamentary seats. (Under the terms of Labour’s rulebook, MPs cannot hold any of the nine posts representing members. If Wolfson or Shawcross were to become members of the Westminster, Welsh or Scottish parliaments, that would thereby create a vacancy on the ruling body that would be filled by the runner-up in the members section, which would likely result in the post being filled by a representative of another faction, hence the self-denying ordnance.)

Shawcroft is facing calls to step down early from the NEC, from Richard Angell, the director of the Corbynsceptic pressure group Progress, and Jennifer Gerber, head of the Labour Friends of Israel. She will be loath to do so as that would mean giving a position on the NEC to Eddie Izzard, who ran on the Corbynsceptic slate, but ultimately even if she is forced to stand down, it will make little difference to the balance of power on the NEC.

More important, though, is what it means for the composition of the vital NEC officers group, which among its wide powers has a vital role to play in selections, particularly selections in parliamentary by-elections. Shawcroft’s role as chair of the disputes panel gave her a seat around the NEC officers table, and although there is a “left” majority in the NEC officers, that is not the same as a majority for the Labour leadership and is different again from a “Momentum majority”. Shawcroft was the only true-blue Momentum representative on that group, with the major power brokers the representatives of three of Labour’s biggest trade unions: Unite, Unison, and the GMB. Shawcroft’s departure may mean that the Momentumites find themselves shut out should a parliamentary seat fall vacant over the next few months.

That will put further pressure on intra-left relations in the Labour party. Shawcroft’s email was only sent to fellow members of the Labour left, and Jeremy Corbyn’s office had already backed sanctioning Bull. The leak, as well as doing further damage to Shawcroft’s reputation, comes at a time when the Labour leadership is under renewed pressure over the party’s failure to deal robustly with anti-Semitism in its ranks. That such unhelpful leaks are coming “from inside the house” as one senior Corbynite put it to me tonight, is a sign that while the Labour left may have won the civil war with the party’s right, its own internal battle may only just be beginning.