The work and pensions secretary says the current system is “ridiculous” and government reforms are simply “common sense”.
Mr Duncan Smith spoke out as disability living allowance (DLA) begins to be replaced by the personal independence payment (PIP), a move criticised by disability rights campaigners.
The new benefit will be phased in, starting in Bootle, covering a few thousand claims a month, and extending to other parts of the country from June.
Working-age benefits and tax credits have also been cut in real terms, as this is the first of three years when rises are to be capped at 1 per cent rises – below the present rate of inflation.
Trials will also begin this month in four London boroughs of a £500-a-week cap on any household’s benefits, and of the new Universal Credit system.
Mr Duncan Smith said the prospect of changes to disability payments had triggered a last-minute surge of applications, with claims more than doubling in some parts of the country.
Ministers say that more than two thirds of claimants currently get the disability benefit for life, without any systematic reassessments to see if they still need it.
He told the Daily Mail: “We’ve seen a rise in the run-up to PIP. And you know why? They know PIP has a health check. They want to get in early, get ahead of it. It’s a case of ‘get your claim in early’.”
He added that rigorous new health checks for claimants were “common sense”, adding: “Seventy per cent of people on it have lifetime awards which means no one sees you ever again. It doesn’t matter if you get better or your condition worsens – it’s quite ridiculous.”
Nearly half a million people have called on the Tory cabinet minister to back up his claim that he could survive off £53 a week for a year in a petition delivered to his office this morning.
Mr Duncan Smith has called the petition as “a complete stunt”.
Musician and part-time shop worker Dominic Aversano, from Twickenham, near London, started the petition on campaigning website Change.org. It has attracted around 450,000 signatures.
Mr Aversano said: “When I started this petition I never imagined the level of support it would get, and the amount of encouragement people would give me.
“It has sent a powerful message to this Government, showing the level of opposition to their vicious welfare cuts.
“Online petitions have become a powerful democratic tool outside traditional political institutions and can turn the tables on those in power.
“Iain Duncan Smith started the week dismissing the suffering of the poor, then he called this petition a ‘stunt’.
“It’s now nearly half a million strong and it’s telling that he continues to ignore such an enormous outpouring of anger and disapproval.”
Disability Rights UK’s chief executive Liz Sayce said: “We are very concerned about the impact of PIP, which could see thousands of disabled people become institutionalised in their own homes.
“For example, the Department for Work and Pensions expects that 428,000 disabled people who currently get the higher rate mobility component will lose it altogether or receive the lower amount.
“This means that many will lose their car under the Motability car scheme so they will no longer be able to get to work or get out and about.”