One in five taxpayers will be paying higher-rate income tax by 2027, in what the Institute for Fiscal Studies describes as a "seismic shift" since the 1990s.

Higher rates of tax were previously thought of as being for the most wealthy, but the six-year freeze in income tax allowances and thresholds that began last month will mean more lower earners are included.

The process is called fiscal drag, and it happens when inflation or income growth moves a taxpayer into a higher tax bracket - increasing tax revenue without the government having to actually raise rates.

The IFS said that by the 2027/28 financial year, more than one in eight nurses, one in six machinists and fitters, one in five electricians and one in four teachers will become higher-rate taxpayers. Almost half of surveyors and legal professionals will also be paying higher rate tax.

Isaac Delestre, a research economist at the IFS, said: "For income tax, the story of the last 30 years has been one of higher-rate tax going from being something reserved for only the very richest, to something that a much larger proportion of adults can expect to encounter."

Mr Delestre said: "Whether or not the scope of these higher rates should be expanded is a political choice as much as an economic one, but achieving it with a freeze leaves the income tax system hostage to the vagaries of inflation - the higher inflation turns out to be, the bigger impact the freeze will have."

In 1991/92, 3.5% of UK adults (1.6 million) paid the 40% higher rate of income tax. By 2022/23, 11% (6.1 million) were paying higher rates, the report said.

The standard personal allowance - the amount someone does not have to pay tax on - is £12,570. Above that amount, a person pays 20% tax until they hit an income of £50,271, when the 40% rate kicks in.

But for the 40% rate to impact the same fraction of people as it did in 1991, the higher rate threshold would need to kick in at nearly £100,000 in 2027/28, the report said.

Scotland's tax bands are different, and the IFS said that it included both those paying the Scottish higher tax rate at 42% and the Scottish top rate at 47% as well as those paying the higher rate of 40% or additional rate of 45% elsewhere in the UK.

A Treasury spokesperson said: "After borrowing hundreds of billions to support the economy during the pandemic and Putin's energy shock, we had to take some difficult decisions to repair the public finances and get debt falling.

"It is vital we stick to this plan to halve inflation this year and get our economy growing again.

"To support working families, we have doubled the tax-free personal allowance, taking three million of the lowest earners out of paying income tax altogether."