The cap - which limits benefits to £500 a week for couples and lone parents and £350 a week for single adults - is a key plank of Mr Duncan Smith's welfare reforms. It is expected to affect about 40,000 households.
The disclosure comes amid reports that Mr Duncan Smith is preparing to announce a tough new conditions on the payment of unemployment benefits at the Conservative Party conference next week.
The Daily Mail reported that the long-term unemployed will be told that they must do an unpaid full-time job or lose their benefits.
The paper said it was expected that claimants who go through the Work Programme - the Government's main back-to-work scheme - but fail to find a job, will be required to take part in unpaid community work or work experience. Refusal to do so could mean the lose of welfare payments.
Mr Duncan Smith told the Mail: "It's not acceptable for people to expect to live a life on benefits if they're able to work."
Critics say that it penalises out-of-work families in areas with high housing charges, forcing them to move out to cheaper areas.
But Mr Duncan Smith said: "Benefits should be a safety net - but not something that gives claimants an income out of reach of many hard-working families.
"We have now successfully delivered a cap on benefits so that out-of-work households know they can no longer claim more than the average family earns and we have returned fairness to the benefits system."
The cap covers the main out-of-work benefits - Jobseeker's Allowance, Income Support, and Employment and Support Allowance - and other benefits such as Housing Benefit, Child Benefit and Child Tax Credit and Carer's Allowance.
It was initially piloted in four London boroughs last April and then rolled out across the rest of the country from July.