The Labour Party lost 8% of its members during 2018, the first official annual drop since Jeremy Corbyn took over as leader in 2015.
The figures, which have been published as part of Labour’s annual accounts, show a fall of more than 45,000 to 518,659 in December last year.
Labour MP Neil Coyle blamed the loss in members on the party’s failure to take a more vocally pro-EU position on Brexit, as well as its handling of allegations of antisemitism.
Mr Coyle said: “Some have denied the drop in membership for months and tried to hide the true figures until now.
“Since this figure was compiled, more members have left and instead of addressing the problem, the leadership are burying their heads in the sand.”
The figure is still significantly higher than the membership of the Conservative Party, which is thought to be around 160,000.
Labour saw more than 300,000 members join the party following the election of Mr Corbyn in 2015 and party rule changes brought in by his predecessor Ed Miliband.
Writing in the accounts, Labour’s general secretary Jennie Formby and treasurer Diana Holland said there had been an increased income from the membership in 2018, adding that the party was in a state of “election readiness”.
“The removal of the Tories outright majority in 2017 and uncertainty around the Brexit negotiations created an opportunity to positively target spending in a non-election year,” they wrote.
The documents also show that Labour had the largest income and expenditure of all the parties in 2018, taking in £45m and spending £46m.
A Labour Party spokesperson said: “Labour is proud to be one of the largest political parties in Europe, with an active and diverse membership who are working to transform our society for the many not the few.
“Our finances are in good health and we remain on an election footing, ready to fight the election whenever it may be called.”
Accounts for the Conservative Party do not give a specific figure for members, but show income from them increased from £0.8m in 2017 to £1.5m in 2018.
The documents also show the Tories took in £34m and spent £36m last year.
Perhaps surprisingly given Brexit, the Liberal Democrats saw a 4% decrease in members throughout 2018.
The figure stood at 93,649 on 31 December, down from 97,403 in 2017, something the report blames on a “lack of political events” last year.
The party treasurer also wrote in the accounts that a “software problem associated with the collection of membership subscriptions” had “significantly impacted” income in 2018.
