A report by the York based Joseph Rowntree Foundation said the so-called bedroom tax - a reduction in housing benefit for social housing tenants deemed to have more rooms than they need - was hitting 100,000 people "trapped" in homes because there was nowhere smaller to move to.
Around 498,000 people had been affected altogether, it said, compared with an initial Government estimate of 660,000 - saving the taxpayer £115 million less in benefits than the £445 expected.
Report author Steve Wilcox said: "The policy is still in its early days, but downsizing and savings have been lower than expected. There are options to alleviate the worst effects of the policy - particularly in the provisions for people with disabilities, bedrooms too small to share, and those unable to move."
The Foundation also produced a report suggesting three quarters of benefit claimants had cut back on food bills and over half were borrowing cash from family and friends or selling possessions to help cover costs as result of the wider squeeze on welfare.