Media playback is unsupported on your device Media caption Khan tells the BBC: Corbyn "has failed to win the trust and respect of the British people"

London Mayor Sadiq Khan has urged Labour voters to ditch Jeremy Corbyn as leader and vote for Owen Smith in the party's leadership contest.

Mr Khan told BBC News Mr Corbyn had "failed to win the trust and respect of the British people".

But a Corbyn campaign spokesman said Mr Khan won the London mayoralty standing on a Labour platform which the party leader backed.

The mayor was, however, "entitled to his opinion", the spokesman added.

Mr Khan said Mr Corbyn had "failed to organise an effective team", and he also criticised his involvement in the EU referendum campaign, saying he "failed to provide the leadership to persuade people to remain in the EU".

"I'm afraid his stance the day after [the referendum] - calling for us to leave the EU immediately - demonstrates to me why I think Owen Smith is the right leader for our party, and why I think he's the best chance we've got to win the next election," he added.

Guide to the Labour leadership election

Matt Wrack, the general secretary of the Fire Brigades Union, described Mr Khan's comments as "disappointing but possibly no great surprise".

"Sadiq comes from that part of the Labour Party that was in government under Blair and Brown," Mr Wrack told BBC News.

"It is disappointing nevertheless. Jeremy Corbyn gave full support for Sadiq in his campaign for mayor, as did a number of unions including my own."

Image copyright AFP/Getty Images Image caption Owen Smith said he was "hugely honoured" to have Sadiq Khan's support

Mr Smith, the former work and pensions spokesman, is challenging Mr Corbyn for Labour's top job, after the leader lost a no-confidence vote of his MPs in June.

MPs who voted against Mr Corbyn have criticised his leadership and his performance during the EU referendum, but supporters have argued that he has been undermined by his opponents in the party.

This week at a hustings event, Mr Corbyn insisted that he could reach out beyond his core supporters and win over "some people that have been tempted to vote Tory".

Mr Khan, who became mayor in May, has until now remained neutral in the leadership contest.

But writing in the Observer, he said Mr Corbyn's personal ratings were "the worst of any opposition leader on record".

"He has lost the confidence of more than 80% of Labour's MPs in Parliament - and I am afraid we simply cannot afford to go on like this," he wrote.

"And you can't just blame a 'hostile media' 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.

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

'Hugely honoured'

Explaining his backing for Mr Smith, Mr Khan added: "On the big issues Owen and I have been on the same side of the argument, including opposing the Iraq War.

"Owen led and - more importantly, won - our fight against the Tories' unfair cuts to tax credits and disability allowances, which would have hurt the most disadvantaged people in our society."

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.

Labour leadership election timetable

19-20 July: The number of MP nominations for each candidate was published

20 July: Deadline for people to sign up as a registered supporter closed

22 August: Ballot papers start to be sent out in the post (Labour Party members only) and by email

21 September: Deadline for ballot papers to be returned is midday

24 September: The result will be announced at a special conference in Liverpool

"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."

Speaking on Friday, Mr Corbyn said his message appealed "all across the country".

He pledged to "win over people... by the policies we put forward", such as decent housing and security at work, including "some people that have been tempted to vote Tory" in the past.

The Labour leadership contest result will be announced on 24 September.