Momentum, the grassroots organisation set up in the wake of Jeremy Corbyn's election victory, has caused panic in the ranks of the Parliamentary Labour Party.
Many Labour MPs view it as a party within a party; a front for the far-left; a vehicle for ousting those opposed to the Corbyn revolution.
So who are Momentum and what do they want?
I went to meet a group of Momentum supporters in Brixton.
They don't easily fit the hard-left stereotype.
There was not a Trotskyite beard to be seen. No one tried to sell me a newspaper.
What Steve, Samira, Nadine and Roland have in common is an enthusiasm fired by Jeremy Corbyn's leadership campaign.
They are Corbyn's foot soldiers.
Steve - a self confessed "middle aged, BBC Radio 4 addict"- says he joined the Labour Party "about 20 seconds" after Mr Corbyn's victory.
Samira also joined after Mr Corbyn won.
"For the first time", she says, "I felt there was somebody I could stand behind and that I could trust".
Momentum currently has about 60,000 supporters, charges no membership fee, organises mostly through social media and has around 50 local groups across the UK.
Its self declared aim is "to create a mass movement for real progressive change".
But the fact anyone can join Momentum has prompted fears it is ripe for infiltration by the Socialist Workers Party (SWP) and other far-left groups who are bitterly opposed to the Labour Party.
Labour MP and former shadow minister Emma Reynolds describes Momentum as "a parallel organisation to the Labour Party".
"I think they have questions to answer," she says.
"What is it they are seeking to do? Who is going to be involved? Are there going to be members of the SWP or Trotskyite, Stalinist parties involved? And there is a suspicion they are really about de-selecting MPs and forcing MPs out of our party."
Some far-left groups like the Trade Unionist and Socialist Coalition (TUSC) have already signalled that they are keen to work with Momentum.
TUSC chairman Dave Nellist - the former Labour MP expelled for his membership of Militant - says Momentum was set up to provide Mr Corbyn with an alternative powerbase with which to purse a left-wing agenda and to "democratise" the Labour Party.
This, he says, would include making the party conference the supreme policy making body, restoring Clause 4 of the party's constitution, committing it to public ownership of industry, and re-introducing mandatory re-selection, seen by opponents as a tactic to purge the party of "moderate" MPs.
It is the issue of re-selection which has, perhaps, caused the most anxiety among Labour MPs.
Back in Brixton, Nadine insists Momentum is not talking about de-selecting sitting MPs.
"But", she says, "that's not to say there won't be opportunities coming up with boundary changes and MPs standing down".
It is clear Momentum wants to bring about change.
Yes it wants to create a broad anti-austerity movement outside of the Labour party.
But it also wants to create a different sort of Labour Party. One that looks more like Momentum; a Corbyn Labour Party.
And that is what frightens so many Labour MPs.