Len McCluskey challenges Keir Starmer and Lisa Nandy to reveal financial backers
Len McCluskey has called on Keir Starmer and Lisa Nandy to provide a full list of those donating to their Labour leadership campaigns.
The Unite boss, who is backing Rebecca Long-Bailey, said there should be "secret financiers sitting in the shadows" in the Labour Party.
His comments came after a YouGov poll for Sky News suggested Sir Keir was on course for a comfortable win in the race to succeed Jeremy Corbyn.
On Wednesday, Ms Long-Bailey's campaign published a list of all the donations of more than £1,500 she had received.
It showed that Unite had given her £200,000, plus an additional £15,000-worth of staffing support.
Speaking at the Oxford Union on Wednesday night, Mr McCluskey urged the Shadow Brexit Secretary's rivals to be equally transparent.
He said: "Trust in politics and politicians is essential, which is why it is really important that all the leadership candidates declare where their campaign money comes from.
"In the Labour Party there should be no secret financiers sitting in the shadows.
"So I appeal to the other candidates, follow Rebecca's lead - publish your donor lists too. It cannot be that we can find out more easily about the millionaires and billionaires happy to open their wallets for Boris Johnson and his Tory party at the black tie ball but we are in the dark about who is funding two possible Labour leader candidates.
"I hope Lisa and Keir will act on this so that Labour really can be the party of transparency and trust."
A spokeswoman for Ms Nandy's campaign insisted that all of her donations were published on the House of Commons register of members' interests.
A source on Sir Keir's campaign told PoliticsHome his campaign was being supported through union funding, online donations and individual contributions; which would also be declared in the register of interests.
The row comes as Labour Party members, registered supporters and affiliates start to receive their leadership ballot papers.
According to the YouGov poll, Sir Keir is on course to receive 53% of first preference, enough to guarantee him victory on the first round of counting.
Rebecca Long-Bailey is in second place on 31%, with Lisa Nandy on 16%.