Labour has lost nearly 26,000 members since last summer, leaked figures reportedly reveal.
More members quit the party last year than the previous six years combined, according to a report by The Times.
More than 15,465 members have left since mid-December, the data seen by the newspaper also shows.
Up to 7,000 members are said to have resigned last month after leader Jeremy Corbyn imposed a three-line whip and ordered his MPs to vote to trigger Article 50 to begin Britain’s exit from the European Union.
The numbers leaving the party could be even higher, according to the paper, as Labour’s systems record active resignations in real time – but lapsed memberships only appear in the data after six months.
It shows total Labour membership at 528,180, down from a peak of 554,000 in July, but still far above the 200,000 members it reportedly had in May 2015.
Official figures are only published in the party’s annual report.
One senior party figure warned the data showed the “tide is turning” against the leadership.
Lord Watts, former chairman of the Parliamentary Labour Party (PLP), told the paper: “I imagine people are losing heart because they can see the polls, they’re talking to their neighbours and people they work with, and are coming to the conclusion Labour is not doing well and, at this point, not convincing the public.”
The party’s humiliating defeat in the Copeland by-election last week prompted calls from some Labour MPs for Mr Corbyn to quit.
Labour’s defeat was embarrassing because it was the first win by the Government over the Opposition in a by-election since 1982, when Labour was also split and led by an unpopular left-wing leader.
Copeland is also a seat which had been solidly Labour since it was created in 1983.
However, Mr Corbyn – who was re-elected in September last year with an increased mandate – remains defiant about his position.
“Now is not the time to retreat, to run away or to give up,” he asserted at the weekend.
Reports of a membership slump come after figures showed Labour raised less money through donations than the Liberal Democrats in the last three of last year.
Mr Corbyn’s party raised £1,970,055 compared with the £1,972,904 donated to Tim Farron’s Lib Dems.
However, it was Theresa May’s Conservatives who took the lion’s share of donations, with £3,610,983.