Brexit is fomenting a significant split in the alliance of Labour left-wing activists that keeps Jeremy Corbyn in power, because of his and the party leadership’s reluctance to commit to hold a referendum on any Brexit deal.

A senior and influential activist told me: “Discussions are under way between leading Momentum activists, anti-Brexit MPs and campaign groups about a new process for drawing up a left slate for this year's NEC election”.

What this means, he said, is that there would no longer be a joint slate of candidates put forward by Momentum and the much older hard-left campaigning group, the Campaign for Labour Party Democracy, or CPLD, which was Corbyn’s ideological and spiritual home for decades.

The influential Labour figure told me: “Prominent activists on the left have privately signalled they will seek to remove any NEC member…who votes against a second referendum” from the section of the ruling NEC that represents constituency Labour parties and their members.

Jeremy Corbyn speaks in Parliament Square in 2016, where the Momentum campaign group held a 'Keep Corbyn' demonstration. Credit: PA

What this means in practice is that Corbyn loyalists could lose control of Labour’s ruling NEC, given that the vote of those who think of themselves as part of the Corbyn project would no longer be unified.

It shows that just like Theresa May’s weakening hold on her own party, Brexit is undermining Corbyn’s control of his party too.

For both parties, Brexit – or in Labour’s case, opposition to it – is trumping party loyalties. An activist said it matters that NEC members should be “people prepared to resist pressure” from Jeremy Corbyn’s office and allies “on behalf of the membership”, which largely wants a confirmatory public ballot on Brexit.

It is highly significant that activists are disclosing their deep concern that many of the nine so-called CLP members of the NEC, who are supposed to represent the membership, may fail to push for a referendum in today’s crunch NEC meeting that will decide the content of Labour’s manifesto for the EU elections.

This split on the left will be a particular dilemma for the Labour veteran Jon Lansman, who founded Momentum and was an important early player in the CPLD.

9.36am update

As a sign of the internal split within Momentum, its official spokesman insisted “nobody is going be removed from the [the movement’s NEC] slate because of their views on Brexit”.

Which I presume is Jon Lansman’s attempt to pour oil on troubled waters. But it is difficult to see how he could enforce a “no recriminations” edict in practice.