The result of the ballot - of more than 550,000 party members and trade unionists as well as people who paid £3 to sign up as supporters - will be revealed at a special conference in central London at 11.30am.
Clockwise from top left: Jeremy Corbyn, Liz Kendall, Andy Burnham, Yvette Cooper
The most Blairite candidate, Liz Kendall, has all but accepted defeat - with the second preferences of her backers potentially playing a pivotal role if none of the candidates secures an overall majority in the first round.
Andrew Smith, the former cabinet minister who gave his signature in to ensure Corbyn had enough votes to join the race, told The Independent that he has 'no regrets'.
Smith, who added his name to back Corbyn with only 10 seconds to go, when the left-winger was one signature short of the 35 needed to make it onto the ballot paper in June. Despite supporting Yvette Cooper, Mr Smith stood by his signature staying it was essential for Labour to have an “open debate” on its future.
Dan Jarvis, the Labour MP who is considered a possible future leader, yesterday became the latest prominent figure to rule out serving under Corbyn in a shadow cabinet due to what he called their "profound" differences on policy. Speaking on Any Questions Radio 4, he said he was "not expecting to get the call' anyway", but would keep supporting Labour if Corbyn won.
The election of Sadiq Khan as the party's candidate for the 2016 London mayoral contest thanks in part to an influx of party activists thought to have been attracted by Mr Corbyn's policies is seen as a pointer towards the result.
But rivals hope figures suggesting far fewer union members have taken part than helped sweep Mr Miliband to a surprise victory in 2010 could yet see the opinion polls confounded.
Mr Khan, who comfortably saw off Baroness Tessa Jowell to secure a tilt at City Hall, was one of the MPs responsible for securing Mr Corbyn a place on the ballot paper only to ensure a wider debate, despite backing Mr Burnham as leader.
Mr Corbyn's radical anti-austerity pitch unexpectedly struck a note with many potential voters and helped inspire a rush of people to sign up for a vote - some 112,799 taking advantage of new rules opening the contest up to non-members.
Fears of "entryism" by political opponents led to around 4,000 being eliminated.
Whoever emerges as the new leader faces a busy first few days, appointing a shadow cabinet, preparing to face David Cameron at prime minister's questions and dealing with the Government's controversial anti-strike laws.
Mr Cameron said victory for Mr Corbyn would be bad for the country as it would break a valuable consensus between the main parties on issues such as nationalisation, nuclear weapons, taxation and union laws
Corbyn's anti-austrity pitch has proved surprisingly popular
"The country is stronger when you have shared objectives rather than when you've got someone who wants to take us back to the days of Michael Foot and Arthur Scargill," he said.
The contest showed that Labour had "completely vacated the intellectual playing field and no longer, in my view, represents working people", he said.
Sources have suggested that only around 50,000 union members have cast their vote, only about a fifth of the number who voted five years ago when Ed Miliband was elected leader.
Under reforms introduced by Mr Miliband, his successor is being elected by a one-member, one-vote system instead of the electoral college which placed more influence in the hands of politicians and unions.
The party will also announce which MP has been elected deputy leader.
Favourite in that race is Tom Watson, who is up against Ben Bradshaw, Stella Creasy, Angela Eagle and Caroline Flint.