How is the Labour leadership decided?

The members of the Labour party get to vote for the next leader. However, the shortlist of candidates will be decided by the party’s 202 MPs (MEPs are supposed to count as well, but it is not clear whether their roles will have been abolished by Brexit by the time a contest is under way).

According to the rule book, the candidates need to get the support of 10% of the combined parliamentary party and MEPs. Each will therefore need to get the backing of about 21 MPs to make it on to the ballot paper – less than the 35 required by Jeremy Corbyn in 2015.

There is also a new requirement set at the party conference last year for a candidate to also secure nominations from 5% of constituency parties or 5% of affiliates from three groupings, at least two of which must be trade unions.

How many members and trade unionists will get to vote?

At the end of last year, there were more than 500,000 members of the Labour party who will be eligible to vote if their subscriptions are up to date. Trade union affiliates will also be able to cast their ballot.

The rules of the contest are decided by the NEC, but the rule book suggests people will also be able to sign up as members before a cut-off date no less than two weeks after the timetable is set and no less than three weeks before ballot papers are sent out. There is no qualifying period of membership prior to the freeze date.

This is likely to trigger a race between the candidates to sign up as many new potential voters as possible before the deadline.

What about registered supporters?

In Corbyn’s first leadership election, more than 100,000 registered supporters who paid just £3 were able to sign up and vote in the contest that saw Corbyn beat Andy Burnham, Yvette Cooper and Liz Kendall. In the next contest, between Corbyn and Owen Smith, there was also a huge 180,000 surge in registered supporters paying £25 to be able to vote.

The party’s rule book says registered supporters will also be able to vote this time, but the Labour website currently only seems to offer three options: full membership at a cost of more than £50 a year; a £26-a-year option for those aged 20 to 26, the unwaged, retired, part-time workers and trade union affiliates; and a £3 annual membership for teenagers, students and members of the armed forces.

How do the members vote?

Voting takes place by post, using the alternative vote system, a type of preferential ballot, in a contest overseen by the national executive committee. If one candidate gets more than 50% of the vote, they are elected. If no candidate manages this, then the candidate who came last is removed and their votes are redistributed to voters’ second-preference candidates. This process is repeated until one candidate has more than half of the votes.