Boris Johnson's tax proposals would cost "many billions" and benefit the wealthy the most, according to the Institute for Fiscal Studies.
Mr Johnson, the front-runner in the race to lead the Conservative Party, has outlined plans to raise the threshold for the higher rate of income tax to £80,000.
The IFS said only 8% of individuals would gain in the short run.
Changes to national insurance would help lower earners, the IFS said.
Under Mr Johnson's proposals:
- the higher rate of tax would be paid on earnings over £80,000 (rather than the current threshold of £50,000)
- the point at which workers start paying national insurance contributions would rise, although a new threshold was not specified
"These are expensive pledges to cut tax [which] between them will cost many billions of pounds", said Tom Waters, research economist at the IFS and co-author of the report.
"It is not clear that spending such sums on tax cuts is compatible with both ending austerity in public spending and prudent management of the public finances," he added.
Following the financial crisis government spending was curtailed in order to reduce borrowing, but the government has recently signalled an easing of austerity.
Mr Johnson's campaign team did not respond to an approach for comment on the IFS report. But earlier this month he told the Telegraph he would fund the income tax cuts partly by using money that had been set aside by the Treasury for a possible no-deal Brexit.
"We should be raising thresholds of income tax so that we help the huge numbers that have been captured in the higher rate by fiscal drag," Mr Johnson said.
However during the televised leadership debate he described the tax plan as "an ambition" rather than a fixed policy.
On Monday, in an interview with the BBC, Mr Johnson addressed questions over Britain's exit from the EU.
The IFS said raising the threshold for the higher rate of tax would take around two and half million people out of the top income tax bracket and cost about £9bn. Top earners would gain an average of nearly £2,500 a year, the IFS said.
But while in the short run only 8% of the population were set to benefit, more people would move into the £50,000 to £80,000 bracket as time went on and would enjoy a lower rate of tax as a result, it said.
The numbers of those falling into the higher tax bracket had "crept up" over time, so that there are currently more than four million higher rate taxpayers, compared with one and a half million 30 years ago, the IFS said.
Raising the point at which workers start to pay national insurance contributions would help low-earning individuals, the IFS said, although it benefited higher earners as well. The IFS said increases to tax credits would be a more effective way to help low income households.
Mr Johnson is running against Jeremy Hunt in a vote by Conservative party members to be chosen as the next Conservative party leader and to take over from Theresa May as prime minister.
Mr Hunt has said he is in favour of a plan to cut the tax on company profits from 17% to 12.5% from 2020.