Keir Starmer's Labour lost an "alarming" 100,000 members in 2021, according to the party's latest annual accounts.
The drop in fee-paying activists resulted in the party's income from membership declining from £19.3 million to £16.1 million in the same year - a fall of over £3 million.
At the end of 2020 total membership stood at 523,332, but fell by over 91,000 to 432,213 in December 2021.
The accounts - published by the Electoral Commission - showed that while the party did increase private donations, it also ended the year with a deficit exceeding £5 million
The left-wing campaign group Momentum, which was set up under Jeremy Corbyn's leadership, described the figures as "alarming".
They said Mr Starmer's failure to stand by the 10 pledges made during the 2020 leadership campaign had "prompted an exodus of Labour members and a financial crisis for the party".
John McDonnell also told The Mirror the "massive wave of resignations" is a financial hit for Labour, but"also a huge political hit as these are the people who are our electoral assets that give us such an advantage over the Tories in getting out the vote".
"There's a message here for the Labour leadership about dissilussionment of so many of our members and supporters," the former shadow chancellor added.
In its report, Labour described 2021 as a "difficult and demanding year" as the country adapted to the impact of Covid, with the party having "difficult discussions and decisions".
"Party financies do remain challenging with pressure on income conciding with increasing costs."
It added: "The action taken by the party to manage costs included reverting to the more traditional mid-election cycle model, after years of remaining on an election-footing.
"The one-off cost of the voluntary severance scheme contributed to the deficit result which required the allocation of cash reserves to fund. For the avoidance of doubt, the party remains debt free".
Hilary Schan, the co-chair of the left-wing Momentum group, said: "Keir Starmer's plege breaking and factional approach has prompted an exodus of Labour members and a financial crisis for the party.
"If Starmer wants to get Labour back on the right track, he should start by properly supporting the affiliated trade unions and striking workers fighting to defend their livelihoods".
But a Labour Party spokesperson said: "Thanks to Keir Starmer’s firm leadership and clear commitment to taking Labour back into power, the party is on track to returning to a firm financial footing – with commercial income and donations rising significantly.”
