Ed Miliband has made an audacious bid to outflank David Cameron on the economy by calling for the 10p rate of income tax to be brought back.
The Labour leader insisted reintroducing the band - controversially scrapped by Gordon Brown - would make society fairer.
In a keynote speech, he said the move could be funded by a new "mansion tax" on homes worth more than £2 million.
The announcement was a surprise inclusion in the address, which had been billed as featuring no significant policy - a prospect mocked by the Prime Minister in the Commons.
Speaking in Bedford - where Tory premier Harold Macmillan delivered his famous "You've never had it so good" line in 1957 - Mr Miliband said: "A One Nation Labour Budget next month would lay the foundations for a recovery made by the many, not just a few at the top.
"Let me tell you about one crucial choice we would make, which is different from this Government. We would tax houses worth over £2 million. And we would use the money to cut taxes for working people.
"We would put right a mistake made by Gordon Brown and the last Labour government. We would use the money raised by a mansion tax to reintroduce a lower 10p starting rate of tax, with the size of the band depending on the amount raised. This would benefit 25 million basic rate taxpayers - moving Labour on from the past and putting Labour where it should always have been - on the side of working people."
Shadow chancellor Ed Balls later said he and Mr Miliband tried to persuade Mr Brown not to scrap the 10p rate in 2007, but were then forced to defend the move under the principle of collective ministerial responsibility after they were over-ruled by the then Chancellor.
Mr Balls said: "Ed Miliband and I said to Gordon Brown before the Budget in 2007 that we shouldn't do this. Clearly we then defended it on the basis of collective responsibility. I said at our conference a couple of years ago that it was a mistake to cut the 10p rate. Gordon Brown has said that. When Ed Miliband and I talked when I first became shadow chancellor, we both said that it was a mistake and we needed to reinstate it and put that wrong right."
A No 10 spokesman said the speech was "a stunning admission of economic incompetence" from Mr Miliband and Mr Balls that their decision in government to scrap the 10p tax rate had hurt millions of working families.