Gordon Brown announced on Monday he would step down as prime minister and leader of the Labour Party later this year.
Here are details of the Labour Party's rules for electing a leader.
The rules governing the election of a Labour leader are complicated and depend on whether there is a vacancy or not.
When the party is in government and a vacancy arises, the cabinet will, after consultation with the party's National Executive Committee, appoint one of its members as interim leader until a ballot can be held.
If the party is in opposition, the deputy leader automatically becomes temporary leader.
To take part in the leadership election, any potential candidate must be supported by 12.5 percent of the members of the Parliamentary Labour Party (PLP).
The voting is split equally three ways between Labour MPs and the Labour Members of the European Parliament; party members; and members of affiliated trade unions who have not opted out of paying a political levy.
The votes of each nominee in each section are then calculated as a percentage of the total votes cast in that section.
The ballot must take place at such a time which means the results can be declared at the annual party conference or at a special leadership election conference.
If any candidate receives a majority of votes, they are declared the leader. If not, the last place contender drops out and their second preferences reallocated, until someone passes the 50 percent figure.
(Reporting by Michael Holden; Editing by Jon Boyle)