Government departments in Whitehall should think more radically about how to ram through £35bn of cuts by the end of the current financial year, or they will be unlikely to implement them, according to the National Audit Office.
The previous Chancellor, Alistair Darling, announced the cuts in the Comprehensive Spending Review of 2007. They comprise about 3% of annual spending on the departments, although current Chancellor George Osborne's intention to slash 25% from the budgets over five years will dwarf this.
Around halfway through Mr Darling's "value-for-money" programme, his government said £10.8bn had been "saved", but the auditors cast doubt on the claims.
The spending watchdog looked again at around a quarter of the claimed savings and said only 38% of the cuts could be defined as sustainable, compared with 44% of cuts where sustainability was deemed uncertain.
The office added that 18% of the reported cuts, rated as "red", were found to have saved no money or had been exaggerated.