More than 580,000 Labour Party members will start voting for Jeremy Corbyn’s successor today ahead of the winner being revealed on 4 April.
Ballots will be sent during the coming week and voters will have until 2 April to decide between Sir Keir Starmer, Rebecca Long-Bailey and Lisa Nandy.
The system used is a preferential vote, so if no candidate wins more than 50% in the first round, then the candidate in last place is eliminated and their votes redistributed until the threshold is reached.
Angela Rayner, Dawn Butler, Ian Murray, Dr Rosena Allin-Khan or Richard Burgon are running for deputy leader.
Each of the three leadership candidates has vowed to offer their rivals jobs in the shadow cabinet if they win.
Sir Keir, the shadow Brexit secretary who is seen as a clear frontrunner, said: “I would happily offer both these candidates a top job in the shadow cabinet and I’d happily serve them if they wanted me to do the same.”
Ms Long-Bailey, the shadow business secretary, an early favourite who has struggled in the contest, said: “I would, I think they’re both brilliant.
“And I would do whatever I was asked and whatever it takes to make sure, if I didn’t become leader, to make sure we were elected as the next Labour government.”
Wigan MP Ms Nandy added: “I like both of these people, I respect them. I would want them in my shadow cabinet doing big jobs.
“But I would equally be happy to serve alongside them in whatever role they chose to give me.”
Mr Corbyn has also said he would be “happy” to carry on serving as a senior member of Labour’s top team.
He said: “I’m happy to serve the party in any capacity because my whole life has been about making my contribution in parliament, holding the government to account and of course speaking out on policy areas.”
Meanwhile, shadow home secretary Diane Abbott said that regardless of who wins she will return to life on the backbenches and work on ensuring the party doesn’t take a “swerve to the right on migration policy”.