The Work and Pensions Secretary has urged wealthy pensioners who do not need to receive benefits to volunteer to hand back the money to the government.
In 2010, the Prime Minister promised to protect universal payments for the whole term of Parliament, and has so far resisted calls to change it.
The Liberal Democrats have proposed a review of the system, and for them to be taxed.
Iain Duncan Smith's appeal comes the day before the start of a pilot project for universal credit, a reform that would bring together several different benefits for those of working age into a single payment.
The shift is designed to ensure that no one is better off unemployed and on benefits rather than in work.
More top news
Lord Montagu, who founded the National Motor Museum at Beaulieu, has died aged 88 following a short illness.
Expensive fares and late arrivals make Britain's railways "second rate" compared to European rivals, a study has found.
Former England rugby captain Will Carling has played down his country's hopes of success at the 2015 World Cup.