Labour Party members are to start voting for Gordon Brown's successor.
Ballot papers have been sent out to members, trade unionists and members of socialist societies, who have three weeks to vote.
Five MPs - backbencher Diane Abbott and former cabinet ministers David and Ed Miliband, Ed Balls and Andy Burnham - are battling to lead the party.
The winner will be announced on 25 September ahead of the Labour Party's annual conference.
Former prime minister Gordon Brown stepped down as Labour leader when the general election resulted in a hung parliament, leading to the UK's first coalition government in 70 years.
Of the five MPs bidding to succeed him, four were cabinet ministers in the previous government. The fifth, left-wing backbencher Ms Abbott, said she was entering the race because there was little to choose from between the other candidates.
Ballot papers will start arriving on Wednesday and party members are expected to vote in large numbers this week, making it a crucial time for candidates to get their message across.
On Tuesday Mr Balls criticised the "soap opera" surrounding brothers David and Ed Miliband, saying members wanted to know whether candidates could deal with the "big issues".
It followed an intervention by former Business Secretary Lord Mandelson who warned Labour could be left in an "electoral cul-de-sac" if its next leader tried to create a "pre-New Labour party".
That was seen as a criticism of Ed Miliband, who had suggested New Labour feared increasing taxes for high earners and had displayed an aversion to Old Labour's anti-Americanism.
But former Labour leader Neil Kinnock, who is backing Ed Miliband for leader, accused Lord Mandelson of "indulging in the sort of personalised factionalism that has inflicted such damage on our party in ancient and modern history".
Ed Miliband told the BBC Lord Mandelson was "entitled to his point of view" but Labour had to "move on" and show "we that we can listen to the country".
Mr Burnham said on Tuesday it was not a "two-horse race" between the Milibands. The former health secretary told the BBC he was "in a strong third position and gaining ground" and said the party had to move on from "the era of self indulgent factionalism".
Meanwhile David Miliband has said he would serve in the shadow cabinet should he be beaten to the leadership by his brother Ed.
Another former Labour leader will be in the spotlight as postal ballots arrive on party members' doormats on Wednesday - Tony Blair is publishing his memoirs. The former prime minister has not formally endorsed any of the candidates but is likely to be asked for his views in a series of interviews as he promotes his book.
Mr Balls, a close ally of Mr Blair's successor Gordon Brown, said it "does not matter a jot" who Mr Blair supports: "Leaders and former leaders write their memoirs and it's about the past... People want to know who is the leader of the future, who with credibility but also with vision can say there's a better alternative to these unfair coalition cuts - that's what I'm doing."
Ms Abbott also told the BBC: "What both Peter Mandelson and Tony Blair forget is that we lost millions of Labour voters in the Tony Blair era and there is no reason to believe that we can get them back by pursuing the same old policies."
Nearly a million people voted in the last contested election, in 1994. Labour has not given an exact figure for the total number of people voting this time round but acting Leader Harriet Harman has said "up to four million people" would be able to vote.