Labour membership falls by 26,000 in six months
Labour has lost 26,000 party members since July, sources have told the BBC.
The number of those who cancelled their direct debit payments or let their subscription lapse represents 4.6% of the party's total 528,000 membership.
Although the majority gave no reason for quitting, party sources said those who did mentioned the party's stance on the EU, which is to back Brexit.
Despite the fall, Labour's total membership is more than 300,000 higher than it was at the 2015 election.
The party enjoyed a surge in membership in the wake of the EU referendum and in the run-up to last year's leadership contest, when Jeremy Corbyn beat Owen Smith.
More than 500,000 people voted in that leadership ballot - 285,000 were party members and the rest registered supporters or union affiliates.
The latest membership figures, first reported by the Times, relate to the period between the leadership contest being called in July 2016 and January this year.
The newspaper said the number of those leaving had accelerated since December and the total figure could actually be higher than 26,000 as those who choose not to renew their subscription take longer to show up on the party's system.
The BBC's political correspondent Iain Watson said sources close to Labour's leadership said many of those who had left in the past six months joined to support Mr Corbyn but found they were denied a vote when the party's ruling national executive retrospectively imposed a qualifying period for membership.
The Labour leader has pointed to the sharp rise in membership since he first ran for the leadership in 2015 as a sign of the strength of the party at grass-roots level and a vindication of the direction he is taking it in.
But divisions over Brexit - with a minority of MPs opposing EU withdrawal - and the loss of the Copeland by-election have raised fresh questions about his leadership. Critics claim Labour is on course for a heavy defeat at the next election due in 2020.