Is the battle to become Labour’s next general secretary turning nasty? Christine Shawcroft, one of Momentum’s directors and one of the nine members of the 39-strong body elected by ordinary party activists, has hit the headlines after posting on Facebook that Labour’s big trade unions were no friends of the Labour left and that ending the trade union link was long overdue.

Shawcroft, who was recently elected chair of Labour’s disputes panel – which adjudicates on whether to dismiss disputes or refer them to the national constitutional committee, the party’s disciplinary body – is a close ally of Jon Lansman, who is competing for the general secretary role against Jennie Formby, a Unite official. Understandably, the general assumption is that the row's cause is down to feelings running high over the contest.

But, having spoken to many of the members of Labour’s ruling national executive committee, last night’s row was not caused by the Lansman-Formby split, but was itself illustrative of one of its causes: that there is a feeling in parts of the Labour left that the trade union movement is not a wholly reliable ally.

The role of the disputes panel is not to decide guilt or innocence, but merely whether someone has a case to answer at the NCC. However, several of those present felt that Shawcroft and other members elected on the Momentum slate wanted to rake over the cases rather than simply referring them upwards. Whether or not that impression is correct, the big trade unions all voted with the Corbynsceptics (or the Labour right or whichever label you like really) on the disputes, and the series of unbroken defeats was a cause of Shawcroft’s frustration.

I’m told that Shawcroft is feeling badly bruised by the remarks, which were made in anger, and that she was surprised at how quickly they took off online.

What does it mean for Lansman’s chances of becoming general secretary? Well, it makes very little difference. As far as the national executive committee’s decision on the next general secretary goes, there are two swing votes: the big trade unions (with the exception of Unite) and the NEC’s Corbynsceptics. For the shortlisting process – which is done by the nine-strong NEC officers group – the big trade unions matter most. Often in Labour history, the hand that controls the shortlist controls the result, as they can manufacture a “cake or death” style choice, in which the full NEC has only the preferred choice of the big trade unions or an unsuitable candidate.

The NEC’s Corbynsceptics are divided as to what their objectives are. For some, there is a feeling that Corbyn has earned the right to do as he wishes, which pushes them towards voting for Formby, considered to be the leader’s office preferred candidate. For others, the aim is to “heighten the contradictions”, as one quipped, which points the towards Lansman. For others it is to preserve the Labour party, which could point in either direction. But as far as that crucial group of swing voters goes, the calculation they will make won’t be based on whether any of Lansman or Formby’s supporters say things they dislike, as they already know they are likely to viscerally disagree with the next general secretary.

What it does do is boost the chances of Paul Hilder, a Corbynite – but one without any faction behind him – or another, as yet unknown person from the left, to emerge as the unity candidate late on.