Disability Benefits: New System Rolled Out
Steven Sumpter from Worcestershire, who suffers from ME and diabetes so finds walking painful, told Sky News he was worried about the future.
Previously, to get disability benefit he had to prove he was unable to walk 50m, but that will be changed to 20m.
He said he fears in the future he will lose half of the money he receives and the subsidised car he relies on.
"It means every single trip to the shops and the doctor will turn into maybe three hours of effort and that will leave me in bed, exhausted and in pain for days afterwards," he said.
The Government insists DLA was outdated and the changes mean those who really need support will now receive it.
Mr Duncan Smith has described the previous system as "ridiculous".
"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 told the Daily Mail.
He added that rigorous new health checks for claimants were "common sense".
Some charities have already expressed concerns that it will mean 600,000 people miss out on support.
Chief Executive of Scope, Richard Hawkes, admitted changes were needed but claimed the Government was motivated by cost cutting.
"The Government has already announced how much the Disability Living Allowance budget is going to be reduced, they've already announced how many people are going to lose DLA and they're introducing a test which is going to provide them with the results they want to reduce those costs.
It’' not right, it's not fair," he told Sky News.
PIP will initially be introduced for new claimants in northwest England, Cumbria, Cheshire, northeast England and Merseyside.
Campaigners handing in the IDS petition at Caxton House, central London