Andy Burnham has called for Tory plans to cut income tax to be scrapped as he set himself at odds with the Labour leadership.
Keir Starmer said Labour would undo the £10,000-a-year Tory tax cut for the 660,000 highest earners in Britain if they win the next election.
Chancellor Kwasi Kwarteng announced the plan to get rid of the 45p top tax rate on earnings above £150,000 a year on Friday in his mini-Budget, which would deprive the Treasury of £2bn a year.
He also vowed to reduce the basic rate of income tax from 20p to 19p in April - a move Labour would maintain if Mr Starmer enters Downing Street.
But Mr Burnham directly contradicted the Labour leadership's position, saying the 1p cut should not go ahead.
The Greater Manchester Mayor also suggested cash earmarked for tax cuts should be refunnelled into a pay deal for nurses, as he branded the Chancellor's mini-budget a "flagrant act of vandalism".
He told Sky's Sophy Ridge on Sunday: "That's my position, I don't think it was a time for tax cuts.
"I think this is a time to support people through a crisis."
Speaking to GB News, he warned of a "mental health crisis" in the winter, and said it would be his "priority" to use the funds earmarked for cutting taxes to help ease the strain.
"I think we're heading into a mental health crisis as well this winter and I would use the money to put it into mental health to ensure nurses have got a fair pay deal, to shore up the NHS," he said.
"That would be my priority."
In another split with the Labour leadership, Mr Burnham threw his weight behind a campaign to back electoral reform, saying the first past the post system ends up "handing inordinate power over our lives to a tiny elite who often don’t reflect the rest of the country."
The former Cabinet Minister, who has run two unsuccessful leadership bids, said the next Labour leader would be a woman "in an ideal world".
But he failed to rule out having another tilt at the top job.
Mr Burnham said: "I wouldn't rule out one day going back, as I've said, I'm just going to be honest about that and I probably am a better politician."
He also took a swipe at party chiefs for failing to grant him a slot to give a conference speech.
Mr Burnham said: "I don't make the decisions, and obviously there's pressure on the conference timetable, but I do think where you've got Labour mayors making real positive changes, such as putting a cap on bus fares, which we've done now and people are benefitting from that, let's showcase that."
In a separate event at The World Transformed festival - running parallel to the Labour conference - Mr Burnham also backed calls for a universal basic income.
The former Labour idea described the policy as an "idea whose time has come".
He said the radical policy would set the "country up to succeed into the next century" and give "everyone enough to cover the basics".
He added: "I think people out there are ready for a different way of running things. They're ready to end the food bank culture.
"They're ready to say lets guarantee the basics for all of our fellow citizens every day of the week when nobody has to worry about keeping warm or being fed."
