Membership for the Jeremy Corbyn-supporting group Momentum has seen its membership swell to more than 30,000, just two years after it was formed.
The campaign organisation started after Mr Corbyn's victory in the Labour leadership race in 2015 and has become highly influential - and controversial - within Labour circles.
The Labour leader had been under pressure to step down as polls showed the party’s popularity at rock bottom, while senior figures also said it would bomb in a general election under his watch.
But Mr Corybn fought off the leadership battle and the party gained ground in the following election – much of it credited to Momentum’s social media campaigning and doorstep canvassing.
Critics of the group, however, claim its activists have taken over constituency Labour parties and agitated against sitting MPs who are critical of the leader.
Momentum said it has added 1,500 new members since Labour's conference in September and that it now has 31,000 activists across 170 local groups, with 15 members of staff.
Co-founder Adam Klug said: "Over the last two years Momentum has become one of the most significant organisations in British politics.
"Our members have breathed life into the Labour Party by getting involved at a local level, running educational events, getting out into the community and supporting workers' struggles across the country.
"The same can't be said for the Tories. The lifeless, moribund Conservative Party on show in Manchester last week was a good example of what happens when a party doesn't empower its members. The intellectual energy disappears, a lack of vision sets in and members begin to desert the party.
"Conservatives should be embarrassed they only have 100,000 members, and we're committed to making sure that ordinary people from across the country continue to join Labour and get involved in their local Labour parties."
It came as a surprise poll found that voters preferred Mr Corbyn as the next prime minister over Theresa May, according to an exclusive survey for The Independent by BMG Research.
It found that 32 per cent of respondents wanted Mr Corbyn in Downing Street compared to 30 per cent for Theresa May.
