Jeremy Corbyn has suffered a strident attack from the Labour Mayor of London Sadiq Khan, who has backed his rival Owen Smith for the party leadership.

Mr Khan enjoys widespread popularity and his election to the mayoralty in May has been the most significant Labour victory under Mr Corbyn.

But relations between the mayor and Labour leader have long been frosty. Now, Mr Khan has come out in favour of a change at the top of the party, giving a much needed boost to Mr Smith.

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 per cent of Labour MPs in Parliament – and I am afraid we simply cannot afford to go on like this."

In an attempted coup which followed the Brexit vote, numerous members of Mr Corbyn’s own Cabinet and other MPs tried to dislodge him as leader. But Mr Khan refused to be drawn into the bitter row, despite being regarded as antagonistic towards Mr Corbyn.

However, Mr Khan’s latest remarks have removed any remaining façade of unity. The country's decision to leave the EU did not sit well with the mayor, who also criticised Mr Corbyn’s performance in the EU referendum campaign.

"Throughout the campaign and aftermath, Jeremy failed to show the leadership we desperately needed. His position on EU membership was never clear – and voters didn't believe him," Mr Khan said in the newspaper. "A third of Labour voters said they did not know where the party stood on the referendum just a week before polling day."

Labour leadership contest: Jeremy Corbyn vs Owen Smith Show all 8 1 /8 Labour leadership contest: Jeremy Corbyn vs Owen Smith Labour leadership contest: Jeremy Corbyn vs Owen Smith Jeremy Corbyn and Owen Smith clash at a leadership hustings in Gateshead, where Mr Smith was scarcely able to answer a question without being booed by Mr Corbyn’s supporters PA Labour leadership contest: Jeremy Corbyn vs Owen Smith “Jeremy himself admitted he was seven out of 10 in terms of his faith in the European Union. He said it,” said Mr Smith during his second live debate with Jeremy Corbyn Getty Labour leadership contest: Jeremy Corbyn vs Owen Smith Ballot papers are currently due to be sent out on 22 August and returned a month later, with the result being announced at a special Labour conference on 24 September Getty Labour leadership contest: Jeremy Corbyn vs Owen Smith Jeremy Corbyn supporters cheer and wave placards as the Labour Leader addresses thousands of supporters in in Liverpool, England Getty Labour leadership contest: Jeremy Corbyn vs Owen Smith Labour Party leadership candidate Owen Smith poses for a picture with supporters during a picnic for young members in London Fields, Hackney in London Getty Labour leadership contest: Jeremy Corbyn vs Owen Smith The Labour leader has a spring in his step at a leadership rally in Sunderland Screenshot Labour leadership contest: Jeremy Corbyn vs Owen Smith Labour leadership contender Owen Smith delivers a speech at the Open University in Milton Keynes, where he promised to reverse Conservative cuts set to leave millions of low paid workers thousands of pounds a year worse off PA Labour leadership contest: Jeremy Corbyn vs Owen Smith Shadow Chancellor John McDonnell has urged Owen Smith to distance himself from those saying they want to split the Labour party Getty

Mr Khan said that it was not enough to blame the media for the outcome of the referendum and "let Jeremy and his team off the hook".

"I know from my own election – up against a nasty and divisive Tory campaign – that if we are strong and clear enough in our convictions, the message will get through to the public," he said.

"That's a test that Jeremy totally failed in the EU referendum. Why would things be different in a general election?"

He urged Labour members to back challenger Mr Smith for the leadership. 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 boon 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.