Ed Balls has pledged to scrap the Tories' tax break for married couples to use the money for the reintroduction of the 10p income tax rate if Labour come into power.
The shadow chancellor described the transferable tax allowance for married couples - due to come into force in April 2015 - as "perverse and unfair".
Mr Balls claimed the flagship Tory tax policy would only be available for a third of married couples, and just one in six families with children.
The plan was announced at the Conservative party conference last year, with Chancellor George Osborne using his Autumn Statement to confirm the intention to introduce the tax break worth up to £200 a year to some couples from April 2015.
The policy was part of a deal with the Liberal Democrats, who had criticised the married couples' tax break in opposition but agreed to the measure in order to press ahead with Nick Clegg's plan to offer free school meals to infant school children.
But Mr Balls told the BBC he would scrap the tax break if he was chancellor after the 2015 election.
Assessing Mr Osborne's tax policies ahead of next week's Budget he said: "Personally, I think that first of all, George Osborne should first of all admit the 50p tax cut at the top was a big mistake.
"But I also think, any tax cut for families on middle and low incomes is better than none. What did he actually announce last year, he said that he would introduce a married couples allowance which, when you look at the detail, only goes to a third of married couples, and one in six families with children, it goes mainly to men.
"We think what we should actually do is scrap the married couples allowance which is perverse and unfair, and use that money to give a tax cut for all middle and lower income families.
"We propose a new 10p starting rate of income tax, it's better than the personal allowance, because it's better for work incentives, it would help two-thirds of married couples, it would help women as well as men, families with children.
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"Let's cut taxes for working families, and let's ease this cost-of-living crisis rather than carrying on pandering to Tory backbenchers with tax cuts that are unfair and don't make sense."
Asked about the prospect of becoming chancellor, Mr Balls said: "I'm daunted, because it's going to be such a task with the deficit we'll inherit. And we've got such challenges as well to get the reforms we need in Europe, in our economy, in our banks, in our energy companies in skills - but it's such an important moment.
"Can Britain earn our way to higher living standards for all? Can we get away from the unfairness of the last few years? I'm up for the task and I'm looking forward to it."
