Andy Burnham received £20,000 from unions as the Mayor of Greater Manchester began to bolster support for a leadership bid.
Three Labour-affiliated unions, all of which called on Sir Keir Starmer to step aside last Friday, handed over tens of thousands of pounds to the Labour candidate for Makerfield between March 24 and April 24.
They said Sir Keir would be forced out of No10 "at some point" and urged the Prime Minister to set out a timetable for his departure.
Mr Burnham received £10,000 on March 24 from the Fire Brigades Union, another £5,000 on April 10 from the Communication Workers Union and a final £5,000 from Unison on April 24, The Times revealed.
The last donation from Unison is understood to help bankroll some of the party's wider local election costs across the borough - rather than specifically to bolster Mr Burnham's campaign.
As one of the UK's largest trade unions, Unison represents some 1.3 million workers in public services across the public and private sector.
Back in December, its General Secretary, Andrea Egan, declared herself to be a "fan" of the Mayor, adding: "I mean, he's King of the North."
Mr Burnham did not receive a donation from the unions in the two years beforehand.
Mr Burnham received £20,000 in donations from unions within a month
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Last Saturday, one of Mr Burnham's closest allies, deputy leader Lucy Powell, delivered a speech at the Fire Brigades Union's annual conference.
There, she vowed Labour leadership would not stand in the way of her colleague's second attempt to return to Westminster.
Ms Powell was the only member of the NEC, Labour's ruling body, to vote in favour of the Mayor's candidacy in the Gorton and Denton by-election in February.
The FBU's leader, Steve Wright, has previously insisted he has not backed a potential candidate before a campaign has been officially launched.
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Union leaders have heaped gushing praise onto Mr Burnham
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He added that Labour-affiliated unions should still have their say in leadership contests.
Meanwhile, the CWU's General Secretary Dave Ward heaped praise on the "King of the North" earlier this month.
He declared Mr Burnham "wants to offer real solutions and wants to put forward solutions to working people".
Previously, the soft-Left politician admitted he supported "unions of all kinds" from the European Union to trade unions.
He said: "People prosper more when they’re part of unions. That’s my belief and I’ll say it clearly."
As part of his campaign this week, Mr Burnham took aim at Margaret Thatcher's legacy in a bid to entice working-class communities.
He slammed the de-industrialisation throughout the 1980s and declared "Britain has been on the wrong path for 40 years".
Unions have recently raised their heads above the parapet to complain that Labour, under the leadership of Sir Keir Starmer, have driven the disillusionment of the working classes.
After the party's disastrous set of local election losses, General Secretary of Unite Sharon Graham said that Labour must change course.
She said: "The writing is on the wall for this Labour Government and it could be the beginning of the end for the party itself.
"The working class have been abandoned and has delivered its verdict.
"They have painted the ballot boxes of our towns and swathes of the Midlands and the North turquoise and even green. They have done so using the brush of decades of Labour failure."
Echoing a similar sentiment, Mr Burnham said: "Labour needs to change.
"What my party has offered in the past has simply not been good enough."