Benefit claimants face "long and uncertain delays" as a consequence of the slow processing of claims under the Government's new Personal Independence Payment, the National Audit Office said.
The head of the NAO Amyas Morse claimed the Department of Work and Pensions did not allow enough time to see if the assessment process could handle a large number of claims.
Mr Morse added:
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.
More top news
A Ukrainian pilot who spent two years in prison in Russia has been sworn into national parliament after being released last week.
The length of time people returning from Zika-hit areas should abstain from having unprotected sex has been doubled by the WHO.
Two young boys have gone missing and their families are "frantic with worry".