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.
Labour leadership hopeful Andy Burnham has called the Institute for Fiscal Studies' verdict on the Budget as "devastating" and condemned Iain Duncan Smith's Commons celebrations.
The IFS said 13 million families will be worse off because of the freeze on working-age benefits.
The Work and Pensions Secretary was seen on camera cheering when the Chancellor announced the new National Living Wage yesterday - a celebration that Burnham said showed the "nasty party was well and truly back".
Since the Budget, a cascade of analysis has crashed across our screens and squirted ink over our newspapers - much of it missing the point.
This afternoon, the Institute for Fiscal Studies presented a damning analysis of George Osborne's Budget. Here we explain the key points.
The Chancellor's "Living Wage" is considerably lower than the rate the Living Wage Foundation campaigns for but this is still a bold move.