Mr Duncan Smith said the new universal credit system was being implemented over four years because "I want to get these things right".
Under the new universal credit, six working-age benefits - income-based jobseeker's allowance, income-related employment and support allowance, income support, child tax credit, working tax credit and housing benefit - will be merged into one.
He said: "We want to say to people, you're claiming unemployment benefit but you're actually in work paid for by the state: you're in work to find work. That's your job from now on: to find work."
Universal credit starts on a limited basis on today for new claimants, who are single, who live in a small number of postcode areas in Ashton-under-Lyne in Tameside, Greater Manchester.
Trials in three more areas - Oldham, Wigan and Warrington - will begin in July.
From October, more claimants will move on to universal credit as and when they have a significant change of circumstances, such as starting a new job or when a child is born.
From April 2014 until October 2017, the rest of those affected in England, Scotland and Wales will be moved on to universal credit in stages. It will start in Northern Ireland from April 2014.
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