The five contenders have been campaigning for four months The Labour Party will learn who their new leader will be as the party conference opens in Manchester.
The winner is expected to be announced at about 1600 BST on Saturday.
David and Ed Miliband, Ed Balls, Andy Burnham and Diane Abbott are in the running to replace Gordon Brown, who quit after Labour lost the election.
Most observers believe it is too close to call between the Miliband brothers, with second and third preference votes likely to prove decisive.
But Ed Miliband was installed as favourite by some bookmakers for the first time on Friday. Ed Balls, Andy Burnham and Diane Abbott are thought to be batting it out for third place.
“I think the priority for all of us will be to unite as a party, and move forward, and to be a credible opposition”
Ed Miliband Labour leadership candidate
An article in the Guardian suggested advisers to the two brothers had held a secret meeting to discuss what role either would play in the other’s shadow cabinet, should one of them lose to the other.
David Miliband, the former foreign secretary who has long been considered the favourite to win, told the BBC he would not quit politics if he lost the contest: “I didn’t run in South Shields in [the general election] in May in order to run away in September. My constituents are very important to me and I’m certainly not walking away from them.”
Asked what job his brother might expect in his shadow cabinet, should he win, Mr Miliband declined to answer saying that would be “tempting fate”.
Ed Miliband, on his way to the conference, told the BBC the priority was for Labour to “unite as a party and move forward and to be a credible opposition. I will do that if I am the leader.”
MPs and MEPs, party members and members of trade unions and allied organisations took part in the vote.
Each of the three components counts for a third of the overall result, but with those taking part casting second, third and fourth preference ballots, it is difficult to predict the outcome.
Meanwhile, Andy Burnham has called for a change in the rules for future leadership contests, with tighter spending limits and a switch to one member one vote instead of separate sections for MPs, trade unionists and party members.
“I don’t see why my vote as an MP is worth 600 times the vote of an ordinary party member. That’s not how we build a mass membership party,” he told The Guardian.
Harriet Harman will return to her job as the party’s deputy leader once the result is announced – she has been acting leader since Mr Brown stepped down as party leader and prime minister in May.
Four of the five candidates are former cabinet ministers – Mr Balls, a close ally of former PM Mr Brown, was schools secretary, Mr Burnham was health secretary and Ed Miliband was the energy and climate change secretary.
Left-wing backbencher Miss Abbott was a late entry to the race who got onto the ballot paper only with the support of David Miliband. But she has used her history as a critic of the Iraq war and other decisions taken by the previous Labour government to differentiate herself from the other candidates.
About three million people are expected to have voted in the contest. Ballot papers are being verified and counted by the independent Electoral Reform Services.
The day after the winner is announced nominations open for the shadow cabinet, with Labour MPs voting to decide who should have a place.
Shadow cabinet candidates have until next Wednesday to put their names into the ring, with about 50 already in the running for the 19 slots up for grabs. The other members of the shadow cabinet – party leader, deputy leader, chairman of the Parliamentary Labour and the chief whip – are voted in separately.
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