Iain Duncan Smith
Darling reveals that while serving as Work and Pensions Secretary, he was told that the programme would be "very difficult" to implement and would cost more than it saved.
The Work and Pensions Secretary tried to pass the buck to the civil service but the NAO report says he never explained how "Universal Credit is meant to work".
New figures show that 320,738 more people are claiming housing benefit than in May 2010.
IDS will appear before the work and pensions select committee on 4 September to answer questions over his misrepresentation of welfare figures.
There's no evidence for his claim that 8,000 people moved into work as a result of the benefit cap but he "believes" it regardless.
Including why an out-of-work family is never better off than an in-work family, why it will cost more than it saves and why it will increase homelessness.
The decision to only provide help with childcare costs to those paying income tax means work will not pay for 900,000 families.
The programme will now apply to just ten job centres, less than 1.5 per cent, this October.
The paper admits it was wrong to state that 878,000 people on incapacity benefit dropped their claims, rather than face a new medical assessment.
Ahead of Duncan Smith's grilling by MPs, a new study shows 92 per cent of claimants are unprepared for the new system in at least one area.