Labour's membership has dropped to around 512,000, according to internal figures shown to the Mirror.
The leaked figures show the party's membership has dipped by more than 50,000 - around 10% - since the highest official estimate of 564,400 in December 2017.
They also show more than 30,000 current members are in arrears, meaning their memberships will lapse too if they fail to pay in a six-month window.
The figures suggest Labour is still more than four times the size of the Conservative Party, which had just 124,000 members in March 2018, and has more than doubled in size since Jeremy Corbyn ran for leader.
But they do reveal a big cut in members and income, despite previous reports of a drop - albeit a much larger one - being rubbished by Labour HQ.
One borough organiser who has lost hundreds of members said some, though not all, left because of the party's stance on Brexit .
The organiser told us "to say we're haemorrhaging members in just one CLP would be an understatement", adding: "The members we’re losing are the long-standing members."
We were leaked the figures after weeks of speculation around membership numbers. Labour is still refusing calls to publish an updated total, despite attacking a flurry of "inaccurate" reports.
After being shown the figures, we put the total membership figure of around 512,000 to four further sources, who all said it was accurate.
The fall may be greater than 10% because reports suggest there was a higher peak in membership just after the 2017 snap election, of around 575,000, that wasn't officially released.
A Labour source today told the Mirror the total number of members had never reached 600,000, as some reports claim, but suggested it had crept into the 570,000s.
A senior Labour insider said: "Everyone knows it's declined. It's quite striking that they talk about resignations being lower than those joining but don't talk about lapses."
The figures we were shown include a regional breakdown.
They say that as of last week there are around 104,000 members in Greater London, 68,000 in the North West and 62,000 in the South East.
There are also around 48,000 members in Yorkshire and the Humber, 44,000 in the South West, 39,000 in the East, 36,000 in the West Midlands, 33,000 in the East Midlands, 26,000 in the 'North' and 26,000 in Wales, the figures show.
Of the existing members more than 30,000 are in arrears, according to the figures.
It comes after Labour said reports of a drop from 650,000 to 500,000 were "simply not true" and "invented".
Blasting ITV political editor Robert Peston, who tweeted the figures, the party's press team tweeted: "Some attention to the basic rules of reporting would help avoid these mistakes."
Labour also condemned reports in the Sunday Times that the party had lost "up to 150,000 members" in recent months.
However, the party has not so far accepted calls to end the speculation and release an updated membership tally.
The figures we have been shown match reports in Scotland's Herald on Sunday, which said Scottish membership had dropped by a fifth to just over 21,000.
Glasgow North East MP Paul Sweeney blamed that drop on a "natural cycle because last year we had a leadership election."
He told the BBC: "As many people who are familiar with party political dynamics will know, many people join parties in order to participate in leadership elections."
After we approached them with the figures today, Labour HQ sources insisted there are always members in arrears, and the number of new joiners has been "particularly high in recent weeks".
"There are always spikes in people joining during general elections, which then lapse in between elections," a Labour source said.
"Membership has more than doubled since 2015 and the Tories won't even divulge their membership figures - estimated at around 70,000. Last year it was revealed that the Conservatives receive more funds from the dead than from living members."
We put our figures, including the regional breakdown, formally to the Labour Party to respond before publication.
A spokesman replied: "Labour is one of the largest political parties in Europe, made up of over half a million members, who are working to transform our society for the many not the few."