Boris Johnson's promise to cut taxes on millions of people on higher incomes would cost £ 9bn and would benefit the richest 10% of UK households the most.

The Institute for Fiscal Studies (IFS) said the leader's proposal in the Conservative leadership race was expensive and possibly incompatible with the Tories' promise to end austerity and secure public finances.

Johnson has announced that he will raise the threshold for income taxes with a higher tax rate, where income is taxed at 40%, from the current £ 50,000 to £ 80,000 if Tory members choose him as leader.

Tom Waters, a research economist at the IFS, said, "It's not clear that such tax cut expenses are consistent with either ending austerity measures in public spending or prudent public finance management."

The proposal met with fierce criticism from across the political spectrum and was widely attacked as a giveaway for the rich, which would exacerbate inequality and damage public finances. Brexit without completion could hurt the revenue of the Ministry of Finance, while tax cuts would be deducted from the funds needed to support the civil service.

Chancellor Philip Hammond has warned the Tory leaders of hopeless tax cuts and spending increases.

According to the IFS, about 4 million high income taxpayers would benefit from Johnson's tax credits, which would bring in almost £ 2,500 each on average. There are 32.75 million British workers and the average salary is 26,400 pounds a year.

About three-quarters of the tax liabilities would go to those in the top 10% of the income distribution, while 97% of the profits would go to the top 30% of the highest incomes.

Johnson has argued that more people pay taxes at the higher tax rate, while the study found that their numbers have risen by 170% since 1990. Johnson would reduce the number of higher-tax-payers by one third to the lowest level since 1990.

The IFS undermined the defense that the cut would inspire aspiring workers and reward the increase in the income pyramid, and stated that only a quarter of UK workers would benefit from the change at any point in their lives or live in a household in which she had someone live. In the short term, only 8% of employees would benefit from the change.

Particularly wealthy pensioners would benefit from it. Johnson said the cuts would be partly funded by increasing the social security contributions of workers benefiting from the income tax reduction. Pensioners, however, do not pay social security.

According to the IFS, over-65s would receive a 60% higher tax cut than those under the age of 65, aggravating inequality between generations.

According to a separate IFS report, middle-income 60 to 74-year-olds already benefit from significantly higher pensions and salaries.

Think Tank Deputy Director Carl Emmerson said the "The Future of Retirement Income" report showed that a combination of generous occupational pension schemes and more people working between the ages of 60 and 74 made up 60% of median income. better off than those in a similar position in the mid-nineties.

The current harvest of retirees is likely to be a bubble with the prospect of less generous pension schemes for younger workers who will be depriving their retirement income in the coming decades.

"Future generations could actually get lower private pensions," he said. However, the employment rates of older people can increase significantly: for example, the employment rates of men between the ages of 60 and 64, who have risen since the mid-1990s, are still well below the quotas of the 1970s, when life expectancy is still high was much lower and health was less good. "