The Government has been urged to improve how it tracks the performance of jobcentres after an official report found that, in many cases, no reason was given for someone moving off employment-related benefits.
The National Audit Office (NAO) said the Department for Work and Pensions (DWP) had simplified its performance measures by showing how many claimants moved off benefits.
Its report said no information was given about how individual jobcentres performed in supporting claimants into a job, adding: "Some may have found work, but in 40% of cases the reason for moving off benefits is not recorded.
"Claimants may have moved on to other benefits, been imprisoned or ceased claiming without taking up work."
A separate survey carried out by the DWP estimated that 68% of claimants left benefits for work.
The NAO said jobcentres had coped well in the face of the economic downturn, which saw a huge increase in the number of Jobseeker's Allowance claimants.
Amyas Morse, head of the NAO, said: "The jobcentre network is a well-established operation and has shown its capacity to maintain services during a downturn. The department can handle changes without major problems in performance or escalating costs. This represents value for money.
"The department must, however, improve how it understands performance, if it is to support claimants effectively. Simply measuring how many people end their claims for benefits does not reveal the true impact of jobcentre services."
The NAO said variations in the caseloads of jobcentres suggested that further efficiency gains could be made.
Jobseeker's Allowance personal advisers had an average caseload of 168 last year.