Thirteen million families will lose an average of £260 each year because of the change to working-age benefits, the Institute for Fiscal Studies (IFS) has said.
Reacting to the first all-Conservative Budget in 19 years, the IFS said it was "regressive" and had taken "much more" from the poor than the rich.
George Osborne earlier defended his Budget, saying it represented a "new contract" for Britain.
Thirteen million families will on average lose £260 each year because of the freeze on working-age benefits, according to the Institute for Fiscal Studies.
Describing George Osborne's Budget as "regressive", IFS director Paul Johnson said it took "much more" from the poor than the rich, adding that after 2017 most benefit rates will have fallen to pre-2008 levels.
"The biggest single cut to welfare spending is set to come from extending the freeze in working age benefits, tax credits and local housing allowance out to 2020," he said.
"That will affect 13 million families who will lose an average of £260 a year."
The IFS also cast doubts on the Chancellor's claim to be delivering a "lower-tax" society, pointing out that the Treasury's own documents show tax increases totalling £6.5 billion a year by 2020.
"Given the array of benefits, it is not surprising that the changes overall are regressive, taking much more from poorer households than richer ones", Mr Johnson added.
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