Shadow energy secretary Ed Miliband has become the first contender for the Labour leadership to secure the 33 official nominations from fellow-MPs needed to get his name on the ballot paper.
The Labour Party's website showed Mr Miliband with 35 nominations, ahead of his brother David - the shadow foreign secretary - on 19.
Shadow children's secretary Ed Balls had four backers listed and shadow health secretary Andy Burnham one, but left-wing backbenchers Diane Abbott and John McDonnell were yet to register a single nomination.
Further nominations are due later.
Nominations formally opened on Tuesday, and would-be leaders have until June 9 to secure the support of the necessary 33 Labour MPs in order to stand as potential successor to Gordon Brown.
Among Ed Miliband's nominators are former Cabinet ministers Hilary Benn, John Denham, Peter Hain, Stephen Timms and Frank Dobson, as well as shadow leader of the Commons Rosie Winterton, shadow transport secretary Sadiq Khan - the first Muslim to attend Cabinet - and up-and-coming backbencher Chuka Umunna, who entered Parliament in this month's election.
Bookmakers' favourite David Miliband has the backing of shadow cabinet colleagues Douglas Alexander and Jim Murphy, alongside senior backbencher Sir Stuart Bell and a former fellow-member of Tony Blair's backroom team Pat McFadden.
Mr Balls' only official nominators so far are Mr Brown's former spokesman Ian Austin, with whom he worked closely in the former Chancellor's Treasury team, ex-ministers Vernon Coaker and Chris Leslie and backbencher Jim Dobbin.
Mr Burnham has secured the nomination of fellow North West MP Paul Goggins.