Voting in the race to replace Jeremy Corbyn as Labour chief ends on Thursday ahead of a new leader of the opposition being announced at the weekend.
Those eligible to vote have until midday to make their choice before the new leader is revealed on Saturday.
Mr Corbyn announced he was quitting after leading the party to its worst election showing since the 1930s in the December snap general election.
Shadow cabinet member Sir Keir Starmer has held a steady lead over rival candidates Rebecca Long-Bailey and Lisa Nandy in polls during the contest.
The latter stage of the leadership race has been overshadowed by the coronavirus outbreak.
The winner of the contest to become Labour’s deputy leader will also be unveiled at the same time.
Shadow education secretary Angela Rayner has been seen as the favourite to take the post of deputy leader.
Ms Rayner is in a race against Dawn Butler, Ian Murray, Dr Rosena Allin-Khan and Richard Burgon.
On February 21 the ballot opened, with voting papers sent out to party members, members of affiliated trade unions and groups and 14,700 “registered supporters” who paid £25 to take part on a one-off basis.
Because of the coronavirus emergency the winners of the two contests will no longer be unveiled at a “special conference” as originally planned.
Instead, the winners will be announced at a scaled-back event with candidates having pre-recorded victory speeches.