The timetable for the contest to succeed Jeremy Corbyn was agreed at a meeting of the party's ruling National Executive Committee today.
A party spokeswoman said that the postal ballot of members will run from February 21 to April 2.
"We are by far the largest political party in the UK with well over half a million members," the spokeswoman said.
"We want as many of our members and supporters to take part, so it has been designed to be open, fair and democratic."
Under the terms of the contest, candidates need to secure the nominations of least 10 per cent (22) of the party's MPs and MEPs.
Those who succeed with then go forward to the second stage when they must get the nominations of five per cent of all constituency Labour parties or three Labour affiliates - of which at least two must be trade unions - comprising at least five per cent of the fully paid up affiliate membership.
The freeze date for new members to join and be eligible to vote will be January 20 in the postal ballot.
This marks a change from the last election in 2016 when a retrospective cut off date was set.
The party said the alteration was intended to ensure the election was as "open and democratic as possible".
As in 2016, people will be able to join as a registered supporter for a fee of £25 provided they are on the electoral register and meet the membership qualification criteria of membership of the party.
The same rules and timetable will apply to the contest to succeed Tom Watson as deputy leader.