The deadline for registration has been moved from noon today to 3pm after people hoping to sign up encountered a series of error messages.
The process to offer registered supporters a vote on the leadership has proved controversial, with a number of MPs calling for a re-run over allegations that it has been exploited by hard-left groups and political opponents including Conservative activists.
Four Labour MPs have called for the election to be halted because of these concerns.
Demands for a “pause” have been fuelled by the surprise emergence of left-wing veteran Jeremy Corbyn as the favourite to succeed Ed Miliband.
But Channel 4 News Political Correspondent Michael Crick wrote yesterday that while Labour could be a victim of so-called “entryism” – infiltration by members and supporters of other parties – this was only a “very small explanation of Jeremy Corbyn’s extraordinary success”.
A YouGov poll on Tuesday gave Mr Corbyn a commanding lead, while bookmakers installed him as favourite to see off Andy Burnham, Yvette Cooper and Liz Kendall.
Some 282,000 party members, 92,000 trade unionists and other affiliated members are eligible to vote, but members of the general public can take part by paying £3 to join Labour as a “registered supporter”, with 70,000 so far signing up.
The rule change was introduced by Mr Miliband after his election as leader and Labour has said it has robust procedures in place to identify and stop “entryists” from voting.
Around 1,200 members or supporters of another party have already been excluded from voting in the leadership contest, including hundreds who stood against Labour in local or national elections.
Labour said the process of stopping bogus supporters from influencing the contest will continue once the deadline has passed. Ballot papers will be sent out from Friday, with the result announced on 12 September,