Supporters of each of the five candidates for the Labour leadership are making a last-ditch effort to secure votes before polling closes.
MPs, MEPs and party members have until 5pm to cast their ballots, and votes are expected to be cast electronically via the Labour website right up until the last minute. Voting for members of trade unions and affiliated organisations closed on Tuesday.
Polls suggest that shadow energy secretary Ed Miliband had closed the gap on brother David as the race came to the wire. But bookmakers Ladbrokes still made shadow foreign secretary David 1-2 favourite on Tuesday night, ahead of his younger brother on 6-4.
Labour's complicated electoral college system and the redistribution of votes to second-preference candidates makes accurate prediction of the result all but impossible.
Around three million people are expected to have voted by the end of today, and their ballot papers will be counted by the independent Electoral Reform Society before the announcement of the successor to Gordon Brown on September 25.
The candidates - also including Ed Balls, Andy Burnham and Diane Abbott - will be informed of the result shortly before the winner is unveiled in Manchester at 4pm on Saturday.
The winner will be plunged straight into Labour's annual conference in Manchester, using his or her first leader's speech to try to rally the party's troops and restore unity after a three-and-a-half month contest which has seen rows over whether to stick to the New Labour agenda.
Meanwhile, Harriet Harman will return to the post of deputy leader, after four months as acting leader.
Voting also closes today in the race for Labour's nomination for the London mayoral elections in 2012, with former mayor Ken Livingstone and ex-MP Oona King believed to be frontrunners. The party's choice will be announced on Friday.
The announcement of the new leader will trigger a ballot for the shadow cabinet, with around 50 MPs expected to put themselves forward when nominations open on Sunday.
