New Labour members have until Monday afternoon to join the party in order to vote in the leadership election.
People need to make applications by 5pm on Monday to be able to take part in the contest.
Five contenders are vying to take over from Jeremy Corbyn as leader of the Opposition by April.
Shadow Brexit secretary Sir Keir Starmer has taken an early edge in the campaign.
The first hustings of the battle were held in Liverpool on Saturday.
Shadow business secretary Rebecca Long-Bailey has attempted to position herself as the main left wing candidate in the contest involving Sir Keir, who is seen to have a more centrist stance.
Backbenchers Jess Phillips and Lisa Nandy are also in the race along with shadow foreign secretary Emily Thornberry.