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
The Bank of England has launched a secret project looking at the economic risk to Britain if the country votes to leave the European Union.
The actress and The Only Way Is Essex star Mark Wright will become husband and wife over the Bank Holiday weekend.
The annual event has earned a place in the record books ahead of the grand final of the 60th anniversary show in Austria this evening.