A member of the ruling committee of Momentum, Jill Mountford, has been expelled from the Labour party, and further members of the committee may face expulsions, sources at Labour party headquarters told the New Statesman.

Mountford has been expelled due to her membership of the Alliance for Workers’ Liberty, a Trotskyite grouping, for which she stood against Harriet Harman, then deputy leader of the Labour party, at the 2010 general election. The AWL is one of a number of “proscribed organisations”, largely to Labour’s left, with which membership of the Labour party is deemed incompatible.

Labour’s Compliance Unit, which is charge of enforcing the party’s rules, is likely to expel further Momentum members from the party. Although people who are not members of Labour are allowed to join Momentum, only Labour party members can hold executive positions within the organisation and members of other parties are banned from membership.

The party’s Compliance Unit has struggled to manage the huge surge in membership, during both the leadership election and after, and many at Labour’s headquarters believe that up to 3,000 of the new membership – Labour now has more than 400,000 members – are members of proscribed organisations.

However, it appears that Momentum, rather than the party proper, is the first target for infiltration, providing an unexpected boon to HQ’s hopes of rooting out proscribed members. Mountford’s own membership went unnoticed by party staffers until she was elected to Momentum’s ruling committee.

There is now a second controversy over whether Mountford forfeits her role on Momentum’s committee. Mountford believes that she retains her position – which may be a bone of contention at the committee’s first meeting later today. Momentum's national committee will rule on whether the expulsion is unreasonable and if they rule that is, will ignore it.

Seperately, Matt Wrack, general secretary of the Fire Brigades Union, has had his attempt to rejoin the Labour party called into question by his local party of Leyton and Wanstead. Wrack, who formerly edited Militant’s magazine for the FBU, Flame, is likely to have his application rubberstamped by the Labour party’s ruling NEC.