London's mayor called on fellow members of Britain's opposition Labour Party to replace leader Jeremy Corbyn, saying his failure to drum up support for Britain remaining in the EU showed he could not command the backing of voters.
Sadiq Khan is one of Labour's most influential members after beating a challenger from the ruling Conservative party to become London's first Muslim mayor earlier this year - a big prize for a party struggling in the opinion polls.
In a boost to lawmaker Owen Smith, who is challenging for the Labour leadership, Khan broke his silence over who he was backing in the contest to say Corbyn could not lead the party to victory at the next election due in 2020.
"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," Khan said in an article he wrote for Britain's Observer newspaper.
"I am afraid we simply cannot afford to go on like this."
Britain's June 23 vote to leave the European Union has shaken the country's political landscape, throwing Labour and the opposition UK Independence Party into turmoil and exposing deep divisions in the ruling Conservatives.
Corbyn, a veteran left-wing member of parliament, was criticised by fellow lawmakers for failing to persuade thousands of traditional Labour supporters to back staying in the bloc and for calling for an immediate beginning to divorce proceedings.
"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?" Khan asked.
Corbyn has denied allegations from his critics that he failed to campaign strongly enough for Britain to stay in the EU and his leadership campaign said Khan should note that he won his election as mayor with Corbyn's help.
"Sadiq Khan is entitled to his opinion," the campaign spokesman said in a statement.
"But he won the London mayoralty, as others have pointed out, by standing on a Labour platform under the leadership of Jeremy Corbyn, backed by the huge numbers who have joined our party to support Jeremy and now campaign for Labour."
Corbyn is favourite to win the leadership contest, supported by thousands of members who swept him to power last year amid a desire for change, particularly among younger voters who face difficulty in getting jobs and owning homes.
Smith is backed by most of Labour's elected lawmakers, dozens of whom have said they have no confidence in Corbyn.
The outcome of the leadership election is expected on September 24.