The Work and Pensions Secretary said there is no “check” on the figures which appear to show almost two-in-three council houses affected by the policy are occupied by someone with a disability.
Tenants deemed to have spare rooms see their housing benefit cut by an average of £14 a week, under the most controversial of all the Coalition’s welfare reforms.
The two-in-three figure is drawn from a DWP equality impact assessment on the policy, which has been cited by the Labour Party and campaigners as evidence that the policy is hitting disabled people the hardest.
Some 63 per cent of those affected have a disability, the study suggests, compared with 66 per cent of all social housing benefit claimants.
However, Mr Duncan Smith said that only a third of those affected receive the Disability Living Allowance.
“The figures you use are figures used for people’s self-declaration of their disability under the Disability Discrimination Act,” he told LBC Radio.
“The fact is all social housing has exactly the same figures. This isn’t just people with spare rooms. The whole of the social housing network, two-thirds of them declare as having some form of disability.”
“I’m not saying they’re right or they’re wrong. I simply say that’s their declaration. There’s no ongoing check. About a third are in receipt of something like DLA, which of itself is a payment to support housing costs.”
Under the Act, the definition of disability covers conditions including dyslexia, heart disease, cancer and stroke. It obliges public bodies to monitor and treat those with disabilities fairly.
The public “very rarely hear the case” for people who are living three to a bedroom and are “desperate” for the under-occupied homes that the policy is intended to free up, Mr Duncan Smith said.
Mr Duncan Smith said he was a “little surprised” to learn that the Liberal Democrats plan to oppose the policy as it stands the 2015 election.
“We were fully engaged in this one for three years, and I didn’t hear anything from them in that time, so I was a little surprised to hear them say they had apparently discovered the secret to this programme, when we had discussed it in depth and detail.”
Separately, Mr Duncan Smith welcomed remarks from Mark Carney, the Governor of the Bank of England, suggesting that tighter benefit sanctions were behind dramatic falls in unemployment. Much of the growth in jobs was among unskilled work, pushing unemployment rates down to 6.4 per cent, the lowest since 2008.
The change “might partly stem from recent reforms to retirement and benefit rules, as well as concerns of households about the need to service debts and provide for pensions,” Mr Carney said.