Shadow Chancellor Ed Balls made the promise as he admitted to being "daunted" by the prospect of being in 11 Downing Street after next year's General Election, given it would be necessary to make further efforts to cut the deficit.
But he stressed: "I'm up for the task and I'm looking forward to it."
Commenting ahead of next week's Budget, Mr Balls said the first thing George Osborne should do was to admit cutting the 50p top rate of tax to 45p had been a "big mistake".
He then attacked the flagship Conservative married tax-break policy, which is due to come into force in April 2015 - just before the election - saying: "What did he actually announce last year? He said 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."
Mr Balls went on: "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."
He added: "Let's cut taxes for working families and let's ease this cost-of-living crisis rather than carrying on pandering to Tory backbenchers."