Former schools secretary Ed Balls has announced he will run for the Labour leadership.

Mr Balls is the third former cabinet member to announce his candidacy, following brothers David and Ed Miliband.

Leftwing MPJohn McDonnell also announced he would stand if he could get enough backers. He tried to challenge Gordon Brown for the leadership in 2007 but could not muster enough support.

Mr Balls, one of Mr Brown’s closest allies, has been expected to announce his candidacy for days.

He made the announcement today while visiting several marginal constituencies in the Midlands and emphasised the importance of hearing “what the country’s got to say”.

But he added: “Whoever wins this I will back them 110 per cent because I think the unity of the Labour Party is vital.”

Former foreign secretary David Miliband remains the bookies’ favourite to win the leadership. In the last week, Harriet Harman, Jon Cruddas and Jack Straw have all ruled themselves out.

Former health secretary Andy Burnham has not yet announced his decision.

Ms Harman, who is acting leader, said yesterday that a successor would not be declared until September, during the party’s annual conference.

Mr McDonnell has criticised contest rules which he said conspired to prevent outside candidates getting enough backers.

Candidates have only until next Thursday to get the required 33 nominations by Labour MPs, a rule which he said prevents “rank and file party members having any say over the process”.

Until he announced his candidacy, research showed Mr Balls had the best gay rights voting record of the candidates.

However, records show Mr McDonnell has voted in favour of every gay equality measure, including gay couples adopting and civil partnerships, since becoming an MP in 1997.

According to Stonewall research of voting on issues such as lesbian fertility rights, civil partnerships, Mr McDonnell scored 100 per cent, while Mr Balls follows on 93 per cent.

Both Milibands are on 86 per cent while Mr Burnham comes fourth with 71 per cent.

Mr Burnham twice voted in favour of a defeated bill which said that those accessing IVF treatment must provide a father figure – effectively discriminating against lesbian fertility rights. He was also absent for three votes on gay couples adopting children.

Hustings for the leadership will take place in June and July, while voting will not open until August. The new Labour leader will be declared on September 25th at the beginning of the party’s annual conference.