In Momentum's internal war, Jon Lansman has won. The group's founder and owner has drawn up a hastily-approved constitution, which will force members to join Labour by 1 July 2017 or face expulsion. Lansman's move is designed to neutralise Momentum's Trotskyist wing, which has long resisted his reforms. Senior members such as Jill Mountford of the Alliance for Workers’ Liberty and former Militant member Nick Wrack, who are barred from Labour, will be forced out.

Mountford said: "This is a coup. We are not splitting and we are not going to be provoked by this. We are going to call a conference for grassroots activists and we will to seek to reverse these changes. The constitution has been imposed, we are going to continue to fight for a democratic organisation."

Lansman's ruthlessness has prompted the rare sight of Blairites praising the veteran Bennite (who I interviewed last year). It was, they wryly noted, the 88th anniversary of Stalin exiling Trotsky to Siberia. "Lansman is behaving like an absolute monarch," said Richard Angell, the director of Progress.

But despite the coming purge, Labour MPs still baulk at the thought of the pro-Corbyn group affiliating to the party. "I will be opposing this with every fibre of my body," vowed Tom Blenkinsop. But what are the chances of Momentum being permitted to affiliate? Under the current rules, the answer is non-existent - unless the group changes beyond recognition. Labour's constitution states that groups which have "their own programme, principles and policy, or distinctive and separate propaganda" or which promote parliamentary or local government candidates (as Momentum does) shall be "ineligible for affiliation to the party."

It is for this reason that other factions, such as Progress, the old right Labour First and the Bennite Campaign for Labour Party Democracy, are not officially affiliated. It is theoretically possible that the the party could change its rules to accept applications from such groups. But in practice it is unlikely that either the NEC or the Labour conference would approve this hazard-strewn move. Even some of Corbyn's NEC supporters are opposed to Momentum's affiliation bid.

Were Labour's rules to remain unchanged, Momentum itself would have to change. It would have to cease to be a pro-Corbyn faction and become a blander socialist society (no disrespect to the Fabians). Rather than a genuine affiliation bid, then, Lansman's announcement looks more like a symbolic declaration. By stating that he wishes to affiliate, he has made it clear that those who don't are not welcome.