Labour has received more than 180,000 applications over the past 48 hours to sign up as registered supporters with a right to vote in the upcoming leadership election.
The 183,541 who each paid £25 to get their vote could play a decisive role in the battle between leader Jeremy Corbyn and challenger Owen Smith, which is also open to those of the party's 380,000-plus members who joined before January 12 as well as affiliated supporters from unions and other organisations.
A spokesman for Corbyn said earlier that it was "reasonable to assume" that the majority of the new registrations come from supporters of the veteran left-winger.
Labour confirmed that Corbyn and Smith will be the only two candidates on the ballot paper for the postal vote which ends on September 24.
By the deadline of 5pm today, Smith had secured the nominations of 162 Labour MPs - more than 70% of the party's representation in the House of Commons - as well as 10 MEPs. Among his nominators were former rival Angela Eagle, who withdrew from the race on Tuesday, and ex-leader Ed Miliband.
The ruling National Executive Committee decided that Corbyn, as incumbent leader, was not required to meet the threshold of 51 nominations to stand.
The party is undertaking a process of vetting the applications for duplicates or people who do not share Labour's values before ballot papers are sent out.