When new group Momentum was launched by Jeremy Corbyn supporters, Labour MPs were immediately alarmed by its decision to allow non-party members to sign up. This, they warned, risked far-left entryism and the creation of a Militant-style "party within a party".

Their fears were given greater credence yesterday by the announcement by the Socialist Workers Party, the most loathed Trotskyist groupuscule, that it intends to participate in Momentum. The SWP's "Party Notes" stated: "There are also various initiatives to re-launch the Labour left. Momentum which has the backing of a group of newly elected Corbyn-supporting MPs such as Clive Lewis and Richard Burgon, looks like it might be the most significant to date (Corbyn and McDonnell have also made supporting statements backing it). It does not seem restricted to Labour members, though it says it will aim to encourage people to join Labour. We should go along to any local Momentum meetings with the aim of taking part as open SWP members, suggesting joint activity, and sign up to be on the email lists. A launch meeting in Manchester last week attracted 70 people, many of them new and comrades had a friendly response when they raised common activity."

For Momentum's Labour supporters, the involvement of the SWP (see Edward Platt's 2014 NS piece for an account of the party's multiple woes) would be a political catastrophe. Indeed, it is precisely because the SWP recognises that its participation would discredit the group that it has adopted this strategy. It intends to support Momentum as the noose supports a hanged man.

It is notable, then, that the group's founders have moved swiftly to repudiate the SWP. An article on Left Futures, the site edited by Momentum director Jon Lansman, declares: "There are extremely good reasons why the SWP and my erstwhile comrades in the Socialist Party should be told to sling their hook when they try and get involved. A passing acquaintance with them is all it takes to understand that they’re fundamentally uninterested in building the wider labour movement, let alone the Labour Party – which is one of Momentum‘s explicit objectives. During the summer the SWP looked upon stormin’ Corbyn with indifference and barely any comment. For the Socialist Party, because Labour was a “capitalist party” Jeremy couldn’t possibly win and it was dead as far as socialist politics were concerned."

After Progress director Richard Angell called on Momentum to "make clear they are opposed to the infiltration of Labour by the SWP and they are neither welcome at their meetings not in their policy making", a spokesperson for the group said: "We oppose the infiltration of the Labour Party by the SWP and we would like to meet with Progress and other groups within the Labour Party soon to build constructive relations."

But the suspicion that Momentum will be infiltrated by hostile left-wingers is likely to endure. If SWP members are to be formally excluded from meetings, the new fear is that its activists go undercover (though it is worth recalling how few there now are). Shadow minister Clive Lewis, a Momentum director, told me this week: "If people are concerned about Momentum, all I would say is judge it on what it does." But for Labour MPs, the jury will remain out for some time.