In his first media appearance since his exit speaking on the BBC's Andrew Marr show, he said the cap was "arbitrarily lowered" following the General Election putting the department of work and pensions "under tremendous pressure".
He said it was "deeply unfair" that cuts were being made to disability benefits while wealthier people were offered tax breaks in the Budget and that he did not know about these tax breaks "until later".
Criticising the government, he said it was too focused on reducing the deficit. "There needs to be a greater, collegiate sense on how decisions are made. This is not the way to do government," he said.
Duncan Smith indicated that he felt increasingly isolated and "semi-detached" from the government and revealed he had considered resigning last year.
He also defended himself against critics who have said his departure was about the EU and not cuts to welfare. Pensions minister Ros Altman this morning blasted Duncan Smith's resignation as "all about the EU"
"This is not some secondary attempt to attack the Prime Minister or about Europe," he insisted.