Momentum’s ruling national coordinating group has voted to withdraw its endorsement of Peter Willsman for Labour’s ongoing elections to its ruling National Executive Committee, putting the organisation – founded to defend and entrench Jeremy Corbyn’s leadership of the Labour Party – on a direct collision course with the Labour leader’s office.

Although voting takes place in two big lumps – once when ballots first arrive on the doorsteps of members, and again right at the end of the voting deadline – it puts Willsman in real danger of losing his seat on the NEC. It is widely expected that Ann Black, who voted for Jeremy Corbyn on both occasions but is seen as independent-minded, will finish narrowly behind the eight candidates endorsed by Momentum. (Essentially everyone involved expects the candidates of Labour’s centre-left, backed by Progress and Labour First, and the comedian Eddie Izzard, a big supporter of Ed Miliband’s leadership and now running as an independent, to finish miles behind Momentum’s slate.) Even a handful of lost votes could see Black take ninth place at Willsman’s expense.

If Willsman’s re-election bid succeeds, however, then nobody – not Momentum, not Jeremy Corbyn, and not the NEC – has the power to compel him to resign his seat, ensuring that the division on the Labour left over how to tackle the party’s anti-Semitism crisis will run and run. In any case, that division has already widened the splits that opened up between the leader’s office and Momentum during the race to fill Iain McNicol’s shoes as Labour party general secretary.

Several Momentum members on Labour’s NEC have tried to bring about a change of approach on the International Holocaust Remembrance Association (IHRA) definition of anti-Semitism, but have been overruled by senior staffers to Corbyn. (Corbyn’s office is, in of itself, split over how to handle the row.)

Jon Lansman, Momentum’s founder and director, spoke out in the WhatsApp group in which Momentum members on the NEC liase with the leader’s office, describing it as a “massive tactical mistake” to take disciplinary action against Margaret Hodge, a senior Jewish Labour MP, for attacking Corbyn on the issue of anti-Semitism, while failing to bring disciplinary measures against Willsman for his remarks on the issue at an NEC meeting. But Karie Murphy, Corbyn’s influential chief of staff, overruled Lansman, saying that Willsman’s apology on the issue meant the matter was closed.

One well-placed source described the row as a “shitshow”. The issue at stake is that Corbyn himself regards the row as a foreign policy issue, confined to the question of how Labour members can talk about Israel, while his critics primarily see it as a domestic issue, confined to the need to reassure British Jews of Labour’s intentions and to take the sting out of the row, which risks derailing a summer of detailed policy interventions from the opposition. Murphy sees the argument as about the right of the Labour leader and the general secretary, Jennie Formby, to make decisions on these matters, and has also criticised John McDonnell, one of Corbyn’s most steadfast allies, for setting out a different approach in public. Both Lansman and Rhea Wolfson, who is standing down from the NEC to run for the seat of Livingston, have been overruled by Murphy on the issue.

Others are worried that Hodge's lawyers, Mischon De Reya, who defended the historian Deborah Lipstadt against the Holocaust denier David Irving, will “take the party to the cleaners” in court. Another group of dissenters are furious that the row is being allowed to distract from a Conservative Party in crisis and a series of meaty policy announcements from the Labour party.

The refusal of the leader’s office to act to take the sting out of the crisis has led to many of the pro-Corbyn commentators, who also liaise with Team Corbyn via WhatsApp, to break ranks and publicly call for Willsman to be stripped of his position on the Momentum slate. Meanwhile, the website Skwakwbox, widely known to be close to allies of Murphy’s, published a defence of Willsman’s conduct.

Although Momentum’s national coordinating group is an elected body with its own mandate separate from Lansman, the reality is that of the 16-strong executive, 14 are closely tied to Lansman and the remainder hold him in high esteem thanks to his long work keeping the Labour left alive and organisationally viable during the long years of New Labour hegemony. Had he wished to save Willsman, his intervention would undoubtedly have done so.

It now puts the Labour leadership on course for two separate confrontations. The first is one with the Parliamentary Labour Party over the IHRA definition, in an argument that could prove the spur for a party split. The second is with Momentum, the leadership’s own Praetorian Guard.