Momentum has banned members of Labour's rival political parties from joining the pro-Jeremy Corbyn pressure group. The move comes after some Labour moderates warned that the organisation could become a mechanism for far-left activists to infiltrate the party.
Concerns were raised after activists from Trotskyist outfits such as the Socialist Workers Party, Alliance for Workers' Liberty and the Socialist Party of England and Wales were found to be attending Momentum's meetings.
The presence of Jon Lansman, who founded Momentum after Corbyn's surprise Labour leadership election, also created controversy. The former Tony Benn fixer wants to reintroduce mandatory-reselection of Labour candidates.
But Momentum has always denied that it is anything like the secretive hard-left group of the Militant Tendency, which had some successes infiltrating Labour in the 1980s.
The organisation has now established democratic structures in the shape of its 52-delegate strong national committee and agreed to set up a 14-member steering committee.
"[The National] Committee strongly affirmed Momentum's identity as a Labour-supporting organisation. As such, the committee voted to exclude supporters of other political parties from Momentum membership or participation in local groups," a spokesman for Momentum said.
He added: "Momentum's network of more than 100 local groups will demonstrate how the organisation can translate a national campaign into local actions and knit together local campaigns into a political message."
The group told IBTimes UK that it sent "five busloads" of activists to the Oldham West and Royton by-election in December, where Jim McMahon was able to retain the seat for Labour. Momentum has since pledged to "mobilise" its supports for the Sheffield Brightside by-election and the May elections across England, Wales, and Scotland.