And the party warned of more help for top earners if the Tories win the General Election.
Shadow chancellor Ed Balls - who is committed to restoring the 50p rate on those earning £150,000-plus a year - challenged David Cameron to rule out a further reduction from 45p if his party holds on to power on May 7.
Labour said its figures showed that in the two tax years since the lower rate was introduced, someone on £1 million a year had seen their bill cut £85,000, a £5 million earner £485,000 and anyone on £8 million almost £985,000.
A further reduction to 40p would benefit them by £340,000, £1.94 million and £3.94 million over the course of the next parliament, it said.
Writing in the Sunday Mirror, Mr Balls, who is MP for Morley and Outwood, said: “While millions are paying more, millionaires are paying less.
“David Cameron and George Osborne have given the very highest earners a huge tax cut. The Tories ditched the idea that we are ‘all in this together’ and looked after their friends first.
“And if the Tories win the election, they’re set to give the top 1% of earners another big tax cut - cutting the top rate of tax again from 45p to 40p.
“How can this be fair when working people are struggling, our NHS is in crisis and when we still need to get the deficit down?
“David Cameron and George Osborne must come clean this weekend. If they carry on refusing to rule out another tax cut for millionaires people will conclude that’s exactly what they’re planning.”
A Conservative spokesman said: “The hardworking many paid the price of Labour’s wrecked economy - with jobs lost, taxes raised, spending out of control and a deficit at record levels.
“Under us, the richest have contributed the most to clearing up Labour’s mess - all while we’ve taken millions out of income tax.
“But with the chaos of Ed Miliband in Number 10, propped up by the SNP, every working family in the country will be hit by a £3,000 tax hike.
“They crashed the economy in the first place, and it’s hardworking taxpayers who’ll pay the price if they’re allowed into power in a month’s time.”