Labour's acting leader, Harriet Harman, today nominated Diane Abbott for the party's leadership.
Ms Harman said she was doing so in the hope of helping to ensure there is a woman on the ballot paper, and will not cast her vote in the election this September.
Her move brings Ms Abbott up to 10 nominations - well short of the 33 she must secure by tomorrow's deadline if she is to join David Miliband, Ed Miliband and Ed Balls in the battle to succeed Gordon Brown.
Ms Harman has made clear that she wishes to remain deputy leader alongside whoever is named leader on September 25.
A source close to Ms Harman said: "The Labour Party knows that whoever wins the leadership contest, there will not be a men-only leadership at the top of the Labour Party as Harriet will continue in her role as deputy leader.
"However, she feels that the party does not want the leadership election and the debate that it will generate to be men only.
"Harriet will play no part in the eventual outcome, as she will not cast her vote. She will continue to serve as deputy leader alongside whoever wins. Therefore, she has nominated Diane."
Shadow education secretary Mr Balls, who already has the 33 nominations from MPs to stand, today said anyone else wanting to add their name to his list of backers should support Ms Abbott instead, to help her get on to the ballot paper.
Mr Balls made his surprise plea during a question and answer session at the GMB union's annual conference in Southport.
The shadow education secretary also signalled his support for Ms Harman's call for half the shadow cabinet to be made up of women.
Ms Abbott is the only woman and the only ethnic minority candidate among the six hopefuls seeking the Labour leadership.
Also hoping to secure their place on the ballot paper are shadow health secretary Andy Burnham, who has so far obtained 26 nominations, and left-wing backbencher John McDonnell with 15.
Leading the field with 71 nominations is shadow foreign secretary David Miliband, who today attempted to woo Labour's MEPs by promising to invite their leader to join his shadow cabinet team if he was elected.
The party's 13-strong representation at the European Parliament could play a significant role in the complicated electoral college system being used by Labour to choose a successor to Gordon Brown.
They form part of the MPs' section in the electoral college, which wields the same voting strength as the other two sections - affiliated organisations including unions and grassroots activists.
Writing on his campaign blog ahead of a visit to Brussels, Mr Miliband said that inviting the MEPs' leader to attend shadow cabinet was an "important and significant" signal that the party was serious about the work of the European Parliament and "wants to lead in a strong, outward-looking European Union".