The Mayor of London endorsed the frontrunner in the race to succeed Jeremy Corbyn in a Twitter message this morning.
He said: “I’ve known Keir for decades. He’s the best person to unite our party, take the fight to the Tories and put Labour in government.”
The endorsement from Labour’s most powerful elected leader will give Sir Keir’s campaign a fresh boost.
I will be voting for @Keir_Starmer to be the next Labour leader. I've known Keir for decades. He's the best person to unite our party, take the fight to the Tories and put Labour in government. #AnotherFutureIsPossible— Sadiq Khan (@SadiqKhan)February 26, 2020
A source said Mr Khan felt that Sir Keir was best placed to unite the party and “make the changes needed in order to win again”.
The two men, both London-based lawyers, used to work together: former human rights solicitor Mr Khan would brief Sir Keir when he was a barrister.
Sir Keir also endorsed Mr Khan to be Labour’s candidate for Mayor of London in 2015.
The MP for Holborn and St Pancras is widely seen as the frontrunner and has endorsements from the overwhelming majority of local Labour groups – many of which previously supported Left-winger Mr Corbyn.
Sir Keir said: “I’m honoured to have received Sadiq’s backing to be the next Labour leader.”
The ballot opened this week, with the winner announced on April 4.