One of the greatest causes of unrest among Labour MPs was the launch last week of new group Momentum. The organisation is billed as a "grassroots movement" to harness the energy of Jeremy Corbyn's campaign. But MPs fear it will become a vehicle for the deselection of critics of the Labour leader as parliamentary candidates. The involvement of Jon Lansman, a veteran of the Bennite Campaign for Labour Party Democracy, and an advocate of mandatory reselection, is the cause of particular anxiety. "When there are selections of an MP, I would like to see MPs who reflect the values of members of the party," he said recently. "The fact is that Liz Kendall got 4 per cent of the votes in the leadership contest."

In my column in tomorrow's NS, I reveal how some are responding to the threat. Frank Field, the chair of the work and pensions select committee, told me that any MPs "picked off" should "cause a by-election immediately" and "stand as independent Labour". He said: "If candidates are picked off they will stand as independent Labour, cause a by-election immediately and a whole pile of us will go down there to campaign for them. They can't expel 60 of us. Momentum ought to know that they're not the only pair of wide eyes in the business. We're not powerless." He added: "Those of us who are not going to let Momentum win have a trump card on our side, which is that we would probably win the by-election." Field's intervention marks the first time since Corbyn's election that an MP has suggested that colleagues could stand against each other.

It is not only critics of the Labour leader who have been antagonised by Lansman. One Corbyn-supporting MP told me: "Jon Lansman needs to wind his neck in and get back in his box. He's doing a lot of damage." Sources suggest that Corbyn, who has rejected calls for the reintroduction of mandatory reselection, may soon distance himself from Lansman.

Momentum supporters cannot fully rebut the claim that it will be used for deselection attempts. Forthcoming boundary changes will force selection contests in some seats and activists can already initiate “trigger ballots” against incumbents. But Corbynites are seeking to reassure their colleagues. Katy Clark, one of Momentum’s six directors, told me: “The reality is, if you’re a good constituency MP, constituency Labour parties recognise that. I would say to anybody who’s worried about new people coming into the Labour Party: embrace it, work with the new members, engage them.” She added: "People need to recognise what took place over the summer. People that voted for Jeremy understood why they were voting Jeremy ... If people supported other candidates they need to reflect on why those candidates weren't successful".

Shadow minister Clive Lewis, another Momentum director, told me: "If people are concerned about Momentum, all I would say is judge it on what it does. If there are people who are spouting off about reselections and so on and so forth, those people are clowns, anyone involved with Momentum that is talking about that is a clown.

"I can speak for myself, I think I can speak for any of the MPs who've been associated with this, we are doing this from a positive perspective in terms of campaigning and engaging those new members - that is it. I want to see as many MPs as possible, as many members as possible involved with it. It has nothing to do with some kind of sectarian project. That's me saying that hand on heart, those are the intentions. That's not the politics that I backed in Jeremy Corbyn - sectarian politics. Always politics, always policy, never personal ... We want this to harness the energy of the Corbyn campaign and work with the Labour Party."