Duncan Smith, who is spearheading a series of welfare reforms for the government, labelled the Child Poverty Action Group (CPAG) action as "ridiculous" and "a massive waste of taxpayers' money" after a judgment on the case was given.
Lawyers acting on behalf of CPAG had argued that the housing benefit changes, which they claim will make a large area of central London no longer accessible to claimants in the private rented sector, are contrary to the scheme's purpose – to prevent homelessness.
They also claimed the government failed to show due regard to the general equality duties under the Race Relations Act 1976 and the Sex Discrimination Act 1975, arguing that ethnic minorities and lone parents will be disproportionately hit by the changes.
A cap on housing benefit means it cannot exceed £250 a week for a one-bedroom property, £290 for a two-bed, £340 for a three-bed or £400 for a four-bed.
After the High Court judged in favour of the government, Duncan Smith said he hoped the charity would "think twice" before taking legal action again.
"CPAG's challenge to our housing benefit reforms was an ill-judged PR stunt, and amounts to nothing more than a massive waste of taxpayers' money and court time," he said.
"The cost of housing benefit has spiralled completely out of control and this judgment is further vindication that our reforms will ensure support is in place for those who need it, but stop the crazy excesses we have seen in recent years of people on benefits living in houses that those in work could not afford.
"I sincerely hope CPAG will think twice before repeating this ridiculous and irresponsible behaviour in future."
Alison Garnham, chief executive of CPAG, said she was surprised by Duncan Smith’s comments.
"The administrative court agreed we had an arguable case and approved the hearing, a judgment that is rightly made by the judiciary independently of ministers. CPAG has a right to use the courts and we will continue to challenge government policy we believe to be unjust and unlawful," she added.
