Jeremy Corbyn said Iain Duncan Smith has "done the right thing" by resigning as he called on Stephen Crabb to reinstate Personal Independence Payments.
In an interview with Sky News, he welcomed the news Mr Duncan Smith has quit as former Work and Pensions Secretary and added: "I wonder where his conscience has been hiding for the past six years."
"I think he (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 (new Work and Pensions Secretary) 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."