The rules of the Labour Party state that "each nomination [for leader] must be supported by 12.5 per cent of the Commons members of the Parliamentary Labour Party." As the number of Labour MPs is 257 (the 258 returned at the general election minus Eric Illsley, who was suspended from the Parliamentary Labour Party) 33 MPs need to support any nomination. Nominations opened on 24 May and were to close on 27 May, but the deadline was extended to 9 June after complaints from John McDonnell, Diane Abbott and Ed Miliband that the short deadline provided insufficient time to secure the 33 nominations from MPs needed for inclusion on the ballot. The ballot took place between 1 and 22 September, with the results announced on the first day of the party's conference in Manchester on 25 September. There were three distinct electorates, the electors of which cast their votes on a "one member, one vote" basis in each applicable category:
Under Labour Party rules, trade unions are allowed to make recommendations to their members, but are barred from doing this in the same communication that contains the ballot paper. During the election, it emerged that both the GMB and Unite had included both an envelope containing the ballot paper, and promotional material for Ed Miliband, their favoured candidate, in the same envelope, attracting criticism that they had breached the spirit of the rules.
At a meeting of the Cabinet held on 10 May 2010, it was agreed that no one would announce their candidacy until after formal negotiations in regards to forming a government were resolved. The Conservative Party and Liberal Democrats formed a coalition on 11 May, and David Miliband became the first person to announce his candidacy the following day. A total of six candidates emerged by 20 May:
Candidates must receive nominations from at least 12.5 per cent of the 257 Parliamentary Labour Party members (33) to appear on the ballot. John McDonnell had 16 nominations when he withdrew on 9 June, in favour of Diane Abbott. The final nominations figures were as follows:
In accordance with the principles of the Alternative Vote system, until one candidate won a majority, the candidate with the fewest votes in each round was eliminated and his or her second preference votes distributed to other candidates.
The total of first-round votes for Balls, Burnham, and Abbott (27.89%) was less than Ed Miliband's vote (34.33%). Thus, it was certain after the first round that Balls, Burnham, and Abbott would all be eliminated; if the Party rules permitted, they could have been eliminated together, reducing the contest to two rounds.
The map below shows the results of the leadership election by constituency. David Miliband took the most constituencies, winning 577 in total. He was followed by Ed Miliband who took sixty-seven constituencies. Andy Burnham won eight seats, all in north-west England, Ed Balls took two constituencies (his own, Morley & Outwood, and that of his wife, Yvette Cooper, Normanton, Pontefract and Castleford), and Diane Abbott won no constituencies. All ties with the exception of Wigan (Andy Burnham and David Miliband) were between David and Ed Miliband. Northern Ireland was counted as one constituency.