IDS will appear before the work and pensions select committee on 4 September to answer questions over his misrepresentation of welfare figures.
The measure proposed by the 40 Group of Tory MPs will do little to reduce the welfare bill, while further stigmatising an already marginalised group.
By denouncing the government for still allowing some families to "live a life on welfare", the party has begun an arms race it can only lose.
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.
The scale of our collective error is startling, as a new survey by Ipsos MORI shows.
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.