The National Audit Office has criticised the government's handling of the introduction of Personal Independence Payments, which will replace Disability Living Allowance for working age people.
A report ‘Personal Independence Payment: early progress' says assessment backlogs are leading to "delays and uncertainty for claimants", this is blamed on the Department of Work and Pensions (DWP) not testing claims volumes.
Amyas Morse, head of the National Audit Office, said: "It is too early to conclude on the Personal Independence Payment programme's overall success and all major programmes run the risk of early operational problems. However the Department did not allow enough time to test whether the assessment process could handle large numbers of claims.
"As a result of this poor early operational performance, claimants face long and uncertain delays and the Department has had to delay the wider roll-out of the programme.
"Because it may take some time to resolve the delays, the Department has increased the risk that the programme will not deliver value for money in the longer term."
The DWP introduced the Personal Independence Payment in a small number of areas from April 2013.
By October 2013, the Department had made only 16% of the decisions expected.
According to the report, by October 2013, there were 92,000 people whose claims were outstanding with assessment providers - Atos Healthcare and Capita Health and Wellbeing - almost three times the number expected at this stage.
In response the Department has said reassessments will not be rolled out nationally, but would be phased in by postcode area.
Among the NAO's recommendations is that the DWP should set out a clear plan for informing claimants about the likely delays they will experience while it works with providers to improve performance.
The report says that the DWP will not achieve the savings it originally expected. Because of the revised timetable for reassessments, savings during this period will fall from £780 million to £640 million.
