More and more people on "relatively modest salaries" are being dragged into becoming higher-rate taxpayers, Budget analysis suggests.
The number of higher rate taxpayers, who pay a chunk of their income at the 40% tax level, could rise from 3.7m last year to 5m by 2014.
The Institute for Fiscal Studies (IFS) made the prediction after studying changes to tax levels in the Budget.
However, lower-income families will benefit from the changes.
In the Budget, the government also decided to end age-related tax allowances for pensioners.
The IFS said that move will cause pensioners to lose 0.25% of their income in 2014.
'Millionaires pay less'
Shadow chancellor Ed Balls criticised the changes to pension allowances.
"The fact is the normal increase in the state pension just keeps up with inflation, but cuts to personal allowances in the Budget will mean 4.4 million pensioners are worse off in real terms," he said.
"It's now even clearer that this was a Budget that asked millions to pay more so millionaires could pay less."
But Chancellor George Osborne told the BBC that no pensioner would be worse off in cash terms, including the "largest increase in the state pension" next month.
"The net changes made by this government, including introducing this triple lock, mean that pensioners are better off."
The tax-free chunk of income, known as the personal allowance, is rising for the under-65s to �9,205 in April 2013.
The IFS said this would cost the Treasury an estimated �3.5bn, and would mean 675,000 fewer people would pay income tax.
From 6 April, people earning taxable incomes of up to �34,370 will pay 20% in tax and people earning between �34,371 and �150,000 are taxed at 40%.