Delays in processing claims under the new Personal Independence Payment have reduced the amount the Government expected to save by £140 million, the National Audit Office said.
The backlog in cases have cut expected savings over the course of this Parliament, with the DWP now forecast to save £640 million a year by 2015, rather than its orginial prediction of £780 million.
Each new PIP claim - worth between £21 and £134 a week to disabled claimants - costs an average £182 to administer, compared to £49 under DLA, the report said.
However, the DWP said it still expects to achieve annual savings of £3 billion by 2018/19, with 3.6 million claims assessed by 2018.
More top news
Disneyland visitors got more than they bargained for when Johnny Depp popped up partway through the Pirates of the Caribbean ride.
United Airlines has reached an undisclosed settlement with a passenger who was dragged off one of its flights.
F1's governing body was paid $5 million after it signed a contract with the sport which favours the top teams, ITV News has discovered.