John McDonnell has withdrawn from the Labour leadership race, to try to help fellow left-wing MP Diane Abbott get onto the ballot paper.
He had 16 of the 33 nominations from MPs needed. Ms Abbott has 11 - but leading contender David Miliband has said he will use his vote to back her.
Mr Miliband, his brother Ed and Ed Balls all have the required support. Andy Burnham was on 31 but his campaign says he now has the 33 needed.
Nominations close at 1230 BST.
Both Ms Abbott and Mr McDonnell are from the left of the Labour party and said they wanted to offer party and union members a choice when they come to vote in the contest in September.
Explaining why he was pulling out of the race, Mr McDonnell said: "I stood for the Labour leadership as the candidate of the Left and trade union movement so that there could be a proper debate about Labour's future in which all the wings of the party were fully represented.
"It is now clear that I am unlikely to secure enough nominations and so I am withdrawing in the hope that we can at least secure a woman on the ballot paper."
He said he had made a last ditch attempt to get the threshold for nominations reduced, but had failed and knew many Labour activists and trade unionists would be "disappointed that their candidate will not be on the ballot".
He urged them to continue to fight for "democracy within the party".
Following his decision Mr Miliband wrote on his page on the micro-blogging website Twitter: "I'm going now to nominate Diane myself. Encourage others to do the same"
Acting Labour leader Harriet Harman nominated Ms Abbot on Tuesday, because she said she did not want to see a "men only" contest.
There has been criticism of the similarities between the leading candidates already through to the next stage of the election - they are all Oxford-educated men in their 40s, who worked as political advisers before becoming MPs and then cabinet ministers.
The chair of Labour's ruling National Executive Committee has written to the chairman of the parliamentary party, Tony Lloyd, underlining "widespread concern" among grassroots party members about the lack of different views being represented.
Ann Black argued that the contest should be wider in terms of "gender, race, political perspective" to generate more interest from party members and give the eventual winner a stronger mandate.
She said the final names on the ballot paper were now "entirely in the hands of Labour MPs". She pointed out that they did not have to vote for the person they nominated and urged them to give "serious consideration to the groundswell of feeling from members and affiliates in the country".
The former education secretary Estelle Morris, now Lady Morris, also told the BBC the leadership contenders were all white, male, Oxbridge-educated and former Downing Street researchers.
"I do think we've got ourselves into a position where it's easier to get to be a Member of Parliament, and once you're a backbencher easier to get to be a minister if you followed that route through politics."
Mr Burnham and Ms Abbott are Cambridge graduates. Ms Abbott was the UK's first female black MP.
Mr Burnham - who was also a researcher, for Tessa Jowell, before becoming an MP, said his background was "different" telling the BBC: "There wasn't much privilege about going to a Merseyside comprehensive in the 1980s, I can assure you of that."
Shadow foreign secretary David Miliband is leading the field with 74 nominations.
Mr Miliband's brother Ed, the shadow climate change secretary, has 57 nominations, while shadow education secretary Mr Balls has 33. Mr Burnham's campaign say he also has 33 - the official list will not be updated until 1230 BST.
Those candidates who get enough nominations will take part in a large number of hustings over the coming weeks.
They will be hoping to persuade those voting that they have the best chance of reinvigorating the party after its election defeat last month.
The leader is chosen by an electoral college system made up of three sections - Labour MPs and MEPs, party members and members of affiliated unions and other affiliated organisations.
People are balloted individually in each of the three sections, with the results from each of the three parts of the electoral college making up a third of the final result
The result will be announced on the first day of the party conference on 25 September.