Labour received more than 160,000 applications to vote in its leadership contest in the final 24 hours of registrations, the party has said.
The number of applications via trade unions more than doubled and the number of party members and £3 registered supporters also rose.
It takes the potential total electorate in the leadership contest to 610,753.
Labour says checks are still taking place on all new members to weed out non-party supporters.
In other developments:
- The registration deadline to vote in the contest was extended after the party's website crashed
- Three leadership contenders complained about a lack of data on voters from party HQ
- Backbench MPs called for the contest to be paused amid fears it was being "infiltrated" by rival groups
The Labour Party has sharply increased in size since May's general election, with tens of thousands of people either becoming full members or registered supporters, thought to be driven to a by a social media campaign by supporters of left wing MP Jeremy Corbyn.
The latest figures, tweeted by the party, are of 189,703 affiliated union members, 121,295 registered supporters and 299,755 members.
Labour said all the numbers were subject to change as it continued to verify all the applications.
In figures released by Labour on Tuesday, the total was 444,000.
At the general election Labour had just over 200,000 full members.
It appears some unions waited until the final day to present their voter applications.
Details of the new supporters signed up through trade unions will be handed to all four campaigns in 10 days' time.
But Mr Corbyn's rivals fear his campaign will already have access to the data, giving him an advantage when it comes to canvassing for votes.
That is because most of those who registered through trade unions are thought to be Corbyn supporters.
All four campaigns are understood to have raised the issue at meeting with party officials on Tuesday, where they sought assurances that there would be a "level playing field", sources say.
The Liz Kendall, Andy Burnham and Yvette Cooper campaigns followed this up with a letter to party HQ asking for the data to be released as soon as possible.
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers will be sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is announced on 12 September
- Who can vote? All party members, registered supporters and affiliated supporters - including those joining via a union
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates
Backbench MPs Graham Stringer, Barry Sheerman, John Mann and Simon Danczuk have meanwhile called for the process to be halted.
Mr Stringer predicted the party would be subjected to the "chaos of legal process".
He told LBC: "I think if the election isn't paused that would be a shame and I think probably that people will take legal action because the rules are not being implemented properly."
Mr Mann said it was a "free for all".
Labour has dismissed such fears, saying "robust" measures are in place to "weed out" bogus applications.
Another MP, Diane Abbott, told BBC News claims of infiltration by members of other parties were "silly", saying Labour was "bending over backwards" to ensure no foul play.
Ballot papers will begin to be sent out on Friday, and the result announced at a special conference on 12 September.
After the deadline passed, Mr Corbyn tweeted support for Labour's "robust selection system in totally new circumstances" and said the party had to united and defeat the Conservatives in 2020.