Labour has experienced a surge in membership since the general election, BBC Two's Newsnight has learned.
Constituency Labour parties (CLPs) have reported rises in local membership amid the ongoing leadership race, in which members will vote from 21 February.
In Hammersmith, west London, the CLP has seen a 32% increase while in Bury North numbers have gone up by 26%.
It is not clear whether these members have only joined to get a vote - but the surge will boost Labour's finances.
The average increase in membership in the CLPs contacted by BBC Newsnight was around 20%.
In Hammersmith, 413 members have joined in a single month and in Bury North 202 more people have signed up.
Elsewhere, Richmond, south-west London, now boasts 350 new members, up 30%, and Hove, East Sussex, has 477 more, up 21%, while Exeter has seen a 25% increase with 300 additional members.
BBC Newsnight has been told the CLP of Sir Keir Starmer, who is a candidate in the leadership race, has seen its membership rise by 1,000 - almost a third.
These are large increases in a short length of time.
Of the CLP chairmen and chairwomen and MPs who spoke to Newsnight, virtually all seemed certain these members joined in order to have a say about the next Labour Party leader.
In 2015 and 2016, the two Labour leadership contests in which Jeremy Corbyn ran, much was made about the "entryism" or programme of enlisting new members to vote for the left-wing candidate.
The intentions of all the new members this time is not certain - there are simply too many of them to be sure.
However, sources within local parties are confident many if not most had joined to vote against the left-wing candidate, a reversal of what happened four years ago.
One MP, in a CLP that now numbers nearly 3,000 members, told Newsnight: "We're just trying to figure it out, but it seems that overwhelmingly the new joiners since the election are moderates who want to vote against Rebecca Long-Bailey in some form.
"A lot joined for Jess Phillips and are now deciding who next."
Another constituency chairman added that in a single ward in his constituency "there are around 100 new members and rejoiners just in my branch".
"We've been phoning them and the overwhelming majority have joined to vote for a moderate candidate," he said.
Another Labour source was convinced this surge meant that the probable outcome of the leadership race was certain.
"I think the contest is over to be honest: [Angela] Rayner [as deputy leader] and Starmer in the first round, unless [Lisa] Nandy really surges," they said.
"A lot of solidly left activists I know are pretty resigned saying things like 'when Starmer wins we'll need to be organised'."
Many CLP officers and MPs believe the party has been flooded by pro-EU members in favour of Sir Keir, someone they deem creditworthy after his advocacy of a second EU referendum.
The increases in Sir Keir's own constituency of Holborn and St Pancras lend weight to this theory.
These new members will be forbidden from taking part in the constituency nomination process, as a result of long-standing Labour Party rules.
However, they will have a vote in the upcoming leader and deputy leader contests.
Such a large number of new voters in this electorate potentially makes the contest more unpredictable.
However, few party insiders believe that this new infusion will benefit Jeremy Corbyn's preferred successor, Rebecca Long-Bailey.
It seems improbable that there are a large pool of left-inclined voters who didn't join for Jeremy Corbyn who would choose to join now.
New members will be also be a financial boon to the party.
But it isn't all good news for Labour.
The one exception to the new surge seems to be Scotland.
Increases for CLPs in Scotland were, in some cases, in single figures - with the prospect of influencing the future of Britain's second biggest party, but Scotland's fourth, apparently failing to excite many politically inclined voters.