The left-wing group Momentum has warned its supporters the group's "future is at risk" amid soaring inflation and a fall in members.
Set up in 2015 to support Jeremy Corbyn's leadership and his radical policy agenda, the group is now launching a month-long funding drive.
In a social media video, Momentum told its supporters on Tuesday it "needs your help" and stressed: "Our future is at risk".
"Without your financial support we won't be able to keep doing what we're doing - and we can't let everything our movement has built disappear".
The group, which is highly dependent on individual member donations, is urging supporters to donate the "price of a coffee" every month.
At the height of its popularity, the organisation had just shy of 30,000 members but it is understood to have fallen by around a third.
A source told The Mirror: "Right now we are struggling, amidst soaring inflation and attacks on the Labour left.
"We're asking our supporters to rally behind us and keep up the socialist struggle for the long-term."
Momentum will now begin a month-long fundraising campaign to prevent the organisation being forced to scale down its activities.
In recent months the group has been highly critical of Keir Starmer and his decision to abandon pledges made in the 2020 leadership contest.
It has also criticised the party's selection process after candidates such as ex-MP Emma Dent-Coad were blocked by the party for standing as parliamentary candidates.
A Momentum spokesperson said: “Across the country, people are crying out for real, transformative change.
"But Labour's leadership is more interested in attacking the Left than harnessing our popular vision and energy.
"So instead, we're building for for the long-term.
"With majorities in Young Labour and Labour Students, strong bases in local government and a crop of rising star socialist Labour MPs like Zarah Sultana and Nadia Whittome, alongside training programmes to identify and train the working-class leaders of the future, the Left has potential for a bright future."
They added: "But like many organisations, right now we're facing serious challenges. That's why we’re asking our supporters to step up and help secure Momentum’s future, so we can keep organising for the socialist future this country needs."
