Cruddas opts to back David Miliband as Ed Balls wins the support of Livingstone

David Miliband won the surprise backing of the Labour left-winger Jon Cruddas last night as his younger brother Ed hit back in a war of words between the two Milibands.

Mr Cruddas nominated Diane Abbott for the Labour leadership to help her to enter the race but has decided to cast his vote for David Miliband. He believes the former Foreign Secretary is the only candidate starting to acknowledge the scale of Labour's defeat in the May election.

The Dagenham and Rainham MP performed strongly in Labour's deputy leadership election in 2007 but rejected calls to stand for the top job when Gordon Brown quit. He hopes to become party chairman.

Mr Cruddas told the New Statesman magazine that David Miliband is "a much more rounded political figure than those who are attempting to caricature him as a one-dimensional Blairite or a late-Blairite." He added: "David is not just going down a checklist of policies; he seems to me to be echoing a more fundamental sentiment, in terms of what Labour needs to do. It's a much more fundamental question of identity that we need to return to."

He appeared to criticise Ed Miliband for attacking the Liberal Democrats for entering a coalition with the Conservatives, saying it was a mistake to do so.

Ed Miliband admitted yesterday that there was "an honest disagreement about the future of the Labour Party" between him and his brother, accusing him of wanting to remain in the "New Labour comfort zone".

The former Energy and Climate Change Secretary rejected David Miliband's suggestion that he wants Labour to pursue a "core vote" strategy rather than woo middle-class voters. He said: "We need to change in order to attract all of these lost voters back to Labour, and that includes the 1.6m voters we lost to the Liberal Democrats, and it also means attracting back the 3m working families whose votes we lost, as well as the more affluent voters.

"We have to change on the economy, on civil liberties and ID cards, and on tuition fees... staying in the New Labour comfort zone will not allow us to reconnect with those we lost."

Ed Balls, the former Schools Secretary, won the backing of Ken Livingstone, the ex-Mayor of London, who said: "He's Labour through and through and is committed to strengthening the trade union link. And he's shown over the last few months that he is the candidate best placed to shorten the life of this Tory-Lib Dem Government."

Tonight Mr Balls will say in a speech: "We need to elect a tough leader of the opposition and someone who can be a credible prime minister too. Party members often say their head tells them they should vote for one candidate but their heart says someone else. But we shouldn't accept such a false and narrow choice. We have to be both radical and credible. And I believe I am the heart and head candidate."