Sadiq Khan has blamed Labour's "catastrophic" defeat on Jeremy Corbyn's leadership and the "repeated failure to tackle anti-Semitism".
The Mayor of London said his party's election performance, which left them with 203 MPs nationwide, had "failed ... the sick, the poorest and the vulnerable".
Writing on Facebook, the former MP for Tooting said:
If we are truly honest with ourselves, we knew in our hearts that Jeremy Corbyn's leadership was deeply unpopular with the British people and that we were extremely unlikely to form a Labour government last night.
Earlier on Friday, Mr Corbyn indicated he would quit as party leader in the early part of next year, and Mr Khan called on the leadership election to happen quickly for the party to "change fundamentally in order to rise to the challenges" of Brexit and the union with Scotland.
This will not be a quick or easy job, and anyone who says they know all the answers today is not credible, whether they are blaming Brexit alone or anything else.