Labour campaign group Momentum claims its membership has risen to more than 35,000 and is continuing to grow by more than 1,000 members per month.
This means the total number of members could eventually become bigger than that of entire political parties.
The Independent has been told by sources from the group that it is already larger than the amount of Ukip members and is likely to become bigger than the 39,000 Green Party activists.
The House Of Commons Library states there are 34,000 members of Ukip, however reports claim the real figure is actually around 25,000 people.
But Momentum is not stopping there as it claims its aim is to recruit more members than the Tory Party by 2022.
The Conservatives do not regularly release membership numbers, however most predictions claim it has around 100,000 paid-up supporters. In 2013 the House of Commons Library reported a total membership of 149,800.
Momentum itself is not a political party, however it is directly affiliated to Labour and those joining it have to become members of the party.
The group is said to be planning on using digital marketing for council election campaigning later this year in an attempt to grow its membership further.