Labour has denied reports that more than a quarter of its members have quit in protest at Jeremy Corbyn’s Brexit policy.
The party said suggestions that up to 150,000 people had left in recent months were “just wrong”.
Newspaper reports claimed the total number of paying members had slumped from 540,000 last summer to around 385,000.
Labour insiders quoted in the Sunday Times said the reported fall had cost the party around £6m.
However, a senior Labour source said: “We are proud of our mass and vibrant membership and claims about this drop off are just wrong.”
While the spokesperson declined to say exactly how many members Labour has, the party’s chairman, Ian Lavery, insisted its membership still numbers more than 500,000 and that it has “more members than all other parties combined”.
Labour is the “largest party in Western Europe” and internal figures “show more joining the party than leaving every week”, he wrote on Twitter.
And Jennie Formby, Labour’s general secretary, said of reports of a slump in members: “This is total fabrication. More people are joining Labour every week than resigning. I’m proud to be general secretary of a mass membership party that is building a social movement for change.”
Mr Corbyn is facing mounting pressure from party members and more than 70 Labour MPs who are urging him to back calls for a fresh referendum on Brexit.
The Labour leader has so far refused, insisting his focus is on securing a better Brexit deal or triggering a general election.
But with 72 per cent of Labour members wanting the public to be given another say on Brexit, frustration in the party over the leadership’s position is growing.
Among Labour voters the figure rises to 78 per cent, according to YouGov polling.
Last week, 71 Labour MPs signed a statement calling on their party leadership to back a fresh vote.
One of them, Bermondsey and Old Southwark MP Neil Coyle, claimed Labour had lost 60,000 members in the last year.
Responding on Twitter to a supporter of Mr Corbyn, he wrote: “Another brainiac who hasn’t worked out yet why 60,000 people left the Labour Party last year…”
Another of the signatories, Tottenham MP David Lammy, warned on Sunday that the party could split over Brexit.
He told Sky News’ Ridge on Sunday: “There is a small group in our party who are so frustrated, who have so much grievance, the fear is that they are going to go off and form another party. I personally reject that.
“But the danger is that, just like 1983, a new party built around a relationship with Europe keeps the Labour Party out of power for a generation.”