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.
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