MPs have expressed "grave concerns" about the ability of Whitehall to deliver efficiency savings without cutting frontline services.
The influential cross-party Public Accounts Committee questioned whether, given their poor performance in the past, departments were "ready" for the scale of the challenge ahead.
Of £35 billion of "value for money" savings demanded by the former Labour government in 2007, only £15 billion had been reported by March this year, it said.
In addition, the National Audit Office found that only 38 per cent of the efficiencies reported by departments had been genuinely sustainable efficiency savings.
In a new report, the committee said: "The inability of departments to improve value for money in a time of increasing budgets casts doubts on government's ability to reduce costs now while minimising the impact on front-line services."
The committee raised particular concern over the Treasury's hands-off role in the delivery of efficiency savings.
"We are concerned at the implication from Treasury that it will simply reduce departments' budgets and then walk away from responsibility for the delivery of the level of savings required across government," it said.