Jeremy Corbyn has asked Iain Duncan Smith "where his conscience has been hiding for the past six years" after his dramatic, sudden resignation over cuts to disability benefits.
The Labour leader said Mr Duncan Smith had "done the right thing" by resigning but said he had presided over "some fairly appalling policies" since becoming Work and Pensions Secretary in 2010.
In an interview with Sky News, he called on Stephen Crabb, Mr Duncan Smith's successor, to re-instate Personal Independence Payments.
He added: "I think [Mr Duncan Smith] has done the right thing to resign, because after all this is a man who has presided over some fairly appalling policies but this latest example of cutting the Personal Independence Payments (PIPs) of a very large number of people ... is shocking."
He added: "He has resigned but I really think the problem is the Chancellor of the Exchequer George Osborne."
Mr Corbyn described the cuts to disability payments as "shameful" and vowed to raise the issue in Parliament next week.
"We have got to defend the principle of Personal Independence Payments at at least the current level and not enable them to take it away from a large number of people," he told Sky.
When asked if he thought Mr Duncan Smith had found his conscience, he replied: "He has taken so much away from people with disabilities and he has suddenly found a conscience now - I wonder where that conscience has been hiding for the past six years."
He added: "The first thing Stephen Crabb has got to do is guarantee a complete reinstatement of PIPs and reinstatement of the independent living fund, and an examination of the appalling way in which people with disabilities go through this availability-for-work tests."