London Mayor Sadiq Khan has backed Sir Keir Starmer to be Labour leader.
The former government minister said Sir Keir was the "best person" to unite the party, "take the fight" to Boris Johnson and return Labour to power.
It comes as Rebecca Long-Bailey revealed her leadership campaign has had a significant cash boost from union Unite and campaign group Momentum.
Unite has spent more than £200,000 on her campaign, while Momentum contributed £120,000.
Unite, which endorsed the shadow business secretary last month, made two separate donations of £100,000 each last month, according to figures published on the MP's campaign website, as well as a payment of £15,000 towards staff costs.
The union was Labour's biggest donor at the 2019 general election, spending more than £3m on the campaign.
Voting in the contest between Sir Keir, Mrs Long-Bailey and Lisa Nandy began earlier this week, with the result to be announced on 4 April.
Announcing his endorsement on Twitter, Mr Khan - who himself faces a battle to be re-elected in May - said he had known Sir Keir "for decades".
He said the MP, who represents the London seat of Holborn and St Pancras, was best placed to take Labour back into government.
In response, Sir Keir said the Mayor of London had shown that "Labour in power can change lives".
I’m honoured to have received Sadiq’s backing to be the next Labour leader.— Keir Starmer (@Keir_Starmer) February 26, 2020
Sadiq is demonstrating that Labour in power can change lives. I look forward to campaigning alongside him to keep London Labour. https://t.co/iMOZ0sKwC8
Mr Corbyn has not publicly backed any of the candidates, although many of his close allies, including Unite general secretary Len McCluskey, are supporting Mrs Long-Bailey.
Figures published by the Long-Bailey campaign show it received £387,478 from two leading trade unions and the pro-Corbyn campaign group Momentum since the middle of January.
The CWU donated just over £52,000 while Momentum, which grew out of Jeremy Corbyn's 2015 leadership campaign and wields considerable influence within the Labour movement, contributed nearly £120,000 towards staff costs.
The Long-Bailey campaign said Momentum's donations were paid out of membership fees and donations to the group from individuals, with 95% of these coming from people giving less than £1,000.
Each campaign is required by law to report donations of more than £1,500 to the Electoral Commission, which publishes quarterly data about political party funding, while the MPs have to declare support on the Commons Register of Members Interests.
By 10 February, when the register was last updated, Sir Keir had recorded a series of small donations from Unison, the UK's largest union which is backing his campaign, totalling just over £8,000.
And Lisa Nandy declared a series of private donations from individuals, the largest of which was £25,000 from Jason Stockwood.