The Labour Party is to appoint its next leader on September 25, during its first annual conference since losing office in the general election.
The ruling national executive committee set the timetable, with nominations by members of the Parliamentary Labour Party opening on Monday.
Acting party leader Harriet Harman said the four-month contest would allow the party to reflect on the outcome of the general election and "re-engage with the British people".
"(The leadership contest) it is going to be a very important opportunity for us to reflect on the result of the general election, to renew the Labour Party and to re-engage with the British people," Harman said.
More than 13,000 people have joined the party since the general election, and the party hopes to increase the numbers further before September 8, the cut off date for new members.
David and Ed Miliband are the only two candidates to have put their name forward in the leadership race so far, with others expected to follow.
Labour said candidates to be leader would declare themselves from May 24 to 27, hustings would take place on June and July and balloting would run from August 16 to September 22.
The winner will be announced at the start of the annual party conference on Saturday September 25, held in Manchester.
Harman said the party would be actively encouraging new members to sign up so they have their say in the election.
"We're asking people to join us and to help shape the progressive future of this country by joining the Labour Party so that they too can play their part in this great election campaign," she said.
The former Commons leader said she had decided not to stand in the contest and instead wanted to return to her role as deputy leader.
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