The most notable intervention during Iain Duncan Smith's emergency statement on Universal Credit (the subject of an excoriating report today by the National Audit Office) came from Alistair Darling.

Darling, who is always listened to respectfully by Conservative MPs, told the House that while serving as Work and Pensions Secretary he was warned by officials that a Universal Credit-style system would be all but impossible to implement and would likely end up costing more than it saved. He said:

I was advised then that it was technically very difficult, if not impossible, to implement at anything like an acceptable cost, and whatever the cost I was quoted it was likely to end up costing an awful lot more.

Darling rightly pointed out the absurdity of Duncan Smith claiming that Universal Credit, which aims to replace six of the main means-tested benefits and tax credits with a single payment, was "on time and on budget" - the NAO report reveals that £34m of IT programmes have been written off and the programme, which was originally due to apply to all new claimants of out of work benefits from this October, will now apply to just ten job centres - and asked Duncan Smith what advice he had received about it. He dismissively replied that he had been told it could be delivered and complained that Labour had just "carped".

In reality, as the NAO report reveals, the DWP is considering delaying the full introduction of the programme until after October 2017 since the current timetable means there will be "less time to deal with any problems identiﬁed during migration".

But while Darling's revelations are damaging for Duncan Smith, they also pose questions for Labour. At the moment the party still supports Universal Credit in principle and has called for cross-party talks with civil servants to save the programme. But if the problems with the system prove as intractable as many fear, Labour may eventually conclude that it is, as Darling's officials warned, simply unworkable.