Labour leader Ed Miliband has called on the Government to "come clean" about why they are funding "a tax cut for millionaires" through the Budget.
Condemning the measures announced by Chancellor George Osborne as "shameful", Mr Miliband vowed that Labour would do its best to prevent the cut in the 50p top tax rate for Britain's wealthiest earners.
He said: "They're funding a tax cut for millionaires and that is totally wrong, it's totally unacceptable and frankly I think the Government is going to have to come clean about why they've done this because they cannot give an explanation about why they're funding a tax cut for millionaires and hitting hard-working pensioners who've worked hard all their lives."
But he stopped short of saying Labour would reintroduce the 50p rate if it returned to government.
Asked if the party would reverse the Chancellor's decision, he said: "We'll set out our tax plans at the election. But I want to stop the 50p tax rate being cut.
"This has not gone through Parliament yet, there has to be a vote in Parliament on this issue...
"This tax cut doesn't come in for a year and we're determined to try and stop it."
He also avoided making a direct promise that Labour would scrap the controversial £1 billion "granny tax" raid by lifting the freeze on the personal allowance for pensioners.
"We're not in favour of the freezing of the personal allowance for pensioners, I think it's a disgraceful move that the Chancellor made," he said.
"He wasn't even open about it in his Budget speech and it's only now in the last few hours that we've discovered the full extent of this tax raid on pensioners."
Speaking as he joined Ken Livingstone, Labour's candidate for London Mayor, on the campaign trail in East Croydon, south London, he was praised for his response to the Budget in the House of Commons yesterday.
Reacting to Mr Osborne's statement, Mr Miliband had challenged Cabinet ministers to raise their hands if they would personally benefit from cutting the 50p tax rate.
Mr Livingstone applauded the Labour leader's "amazing performance" in showing up "all those frontbenchers who didn't want to put their hands up".
He condemned the Budget as one that "hit the poorest and given the most to the richest".