Sadiq Khan has decided to back Owen Smith for the Labour Leadership.
The London Mayor had previously indicated that he wouldn't take sides in the contest, but writing for the Observer he is devastating about Jeremy Corbyn saying: "By every available measure, if Jeremy remains as leader, Labour is extremely unlikely to win the next general election.
"The hopes of the members who have joined our party would be dashed again. 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's MPs in Parliament - and I am afraid we simply cannot afford to go on like this."
So Owen Smith now has the backing of Labour's most powerful elected politician, and he needs it.
Although he is persuading Labour members to back him, Jeremy Corbyn is still thought to be much more popular amongst the Party membership and Registered Supporters.
Mr Corbyn often reminds us that every Parliamentary by-election and the London Mayoral election have been won under his leadership.
But most of the winners of those elections and now the London Mayor himself are backing Owen Smith.