Prime Minister Boris Johnson's tax plans would cost up to £20 billion a year and mostly benefit the richest people in the UK, according to research by Britain's leading economic think tank.

The Institute for Fiscal Studies said Johnson's tax plans would likely be ineffective and expensive and provide little benefit to people with lower incomes.

When he took office in July, Johnson promised a radical overhaul of the UK's tax system.

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LONDON — Prime Minister Boris Johnson's proposed tax cuts would cost the government up to £20 billion and mostly benefit the richest households in Britain, according to new independent research.

The Institute for Fiscal Studies warned on Friday that the prime minister's plans to cut taxes for higher earners and lift the threshold for National Insurance contributions would likely be ineffective and costly and said there were "much better" ways to cut taxes and support lower earners.

When he took office in July, Johnson promised a radical overhaul of the UK's tax system. He's still committed to raising the threshold for the 40% "higher rate" of income tax to £80,000 from £50,000. He has said that raising the National Insurance contribution threshold would help support lower earners.

The IFS found that raising the higher-rate threshold to £80,000 would cost about £8 billion a year in lost tax revenue.

Johnson has not specified how far he would lift the threshold for National Insurance contributions, but the IFS estimated that every £1,000 increase would cost £3 billion to £5 billion.

John McDonnell, Labour's shadow chancellor, said the research showed that the government "cannot be trusted with our public services."

"These proposals to raise NIC and income tax thresholds will rip out £10 to £20 billion a year from our already decimated public services, according to the IFS, and put those billions into the pockets of the rich," he said.

Raising the National Insurance contribution threshold would only minimally benefit the lower earners it's designed to help, with only 3% of gains going to the poorest 20% of households, the IFS said.

If you are going to spend that much cutting taxes for those on high incomes or supporting low earners, you could find much better ways of doing so than the policies proposed by the prime minister.

A £3 billion cut to National Insurance contributions would raise incomes of that group by just £20 a year on average, a figure the IFS said "would pale in comparison with the losses from benefit and tax credit cuts experienced by this group since 2010, which have reduced their average incomes by more than 10%."

Raising the higher-rate threshold to £80,000 next year "would benefit the highest-income 8% of adults and 13% of income taxpayers," the IFS said, adding that "15% of adults live in a household where someone would gain immediately."

Xiaowei Xu, a research economist at IFS and an author of the research, said that "given promised spending increases and growing demographic pressures, taxes are likely to have to rise over the next decade."

"Promising £10 billion to £20 billion of tax cuts is therefore a major commitment," Xu added.

"If you are going to spend that much cutting taxes for those on high incomes or supporting low earners, you could find much better ways of doing so than the policies proposed by the prime minister."