Left-wing Labour backbencher Diane Abbott squeezed onto the starting line for the Labour leadership this afternoon just before nominations closed.

She will contest four shadow cabinet members for the post – Ed Miliband, David Miliband, Ed Balls and Andy Burnham.

A sixth hopeful, the left-winger John McDonnell, stood aside yesterday in order to allow Ms Abbott to become the only woman and left-winger in the race.

She won the 33 nominations required shortly before 12.30pm.

The party’s new leader will be announced on September 25th after Labour members, MPs, MEPs and trade unionists are balloted.

Ms Abbott, who was the first female black MP, said she decided to join the contest to improve its diversity. Her four rivals are all Cambridge or Oxford-educated white men in their forties.

Acting Labour leader Harriet Harman said “Over the next few months over four million people will have the chance to help shape Britain’s progressive future by choosing the next leader of the Labour Party.

“This will be the biggest and most widespread election of any political party or any organisation in this country. The contest will be open engaging and energising. It will be a chance to invite supporters to join the party to have a vote.”

In the run-up to the close of nominations, gay charity Stonewall ranked each candidate on their gay voting record in the last five years.

Mr Balls, the former education secretary, has the strongest record with a rating of 93 per cent. Both Milibands are on 86 per cent.

Ms Abbott has a rating of 79 per cent.

Although she has spoken in favour of gay rights and been a judge in the Stonewall awards, she has missed more than half of the gay rights votes she could have attended since 1998.

She has never voted against gay rights measures, but was absent for important votes on gay couples adopting, civil partnerships and equalising the age of consent.

Former health secretary Mr Burnham comes last with 71 per cent.

In 2008 he twice voted in favour of amendments that sought to discriminate against lesbian couple’s rights to access IVF treatment. He abstained on three votes relating to the rights of gay couples jointing adopting children.