London mayor Sadiq Khan has come out and urged Labour members to ditch Jeremy Corbyn and support Owen Smith for leader.
Mr Khan, whose success in the capital last May was Labour's best result in an otherwise disappointing set of local elections, said Mr Corbyn had lost the confidence of Labour MPs and failed to win over the public.
Writing in The Observer, he said: "Jeremy has already proved that he is unable to organise an effective team and has failed to win the trust and respect of the British people.
"Jeremy's personal ratings are the worst of any opposition leader on record and the Labour Party is suffering badly as a result.
"He has lost the confidence of more than 80% of Labour MPs in Parliament – and I am afraid we simply cannot afford to go on like this."
Having previously refused to say which of the two candidates he was supporting, Mr Khan's decision to come out for Mr Smith will be seen as a huge boost for the former shadow work and pensions secretary who is widely thought to be trailing in the contest.
Mr Smith said he was "hugely honoured" to have Mr Khan's support.
"Sadiq ran a fantastic campaign to win power for Labour in London this year, securing a huge mandate from party members and the electorate," he said.
"He showed that a vision of hope and optimism can win, if it's backed up with a credible plan to deliver real meaningful change for people's lives.
"Since that election we have already seen what a difference Labour can make when we hold power."