Fraud and error in benefit payments remains "unacceptably high" at £3.3 billion, according to a new report.
The figure, for 2013/14, equates to 2% of the total forecast benefit expenditure of £163.9 billion.
Amyas Morse, the Comptroller and Auditor General, announced he could not sign off the accounts of the Department for Work and Pensions (DWP) because of the level of fraud and error.
He has qualified his audit opinion, a move which has been made every year since 1988/89.
The figure of £3.3 billion is slightly down on the £3.5 billion of the previous year, which was mainly caused by the removal of council tax benefit from the estimated total.
The National Audit Office said in today's report that the DWP needed to understand how and why overpayments arose in individual benefits so it could develop effective ways of reducing fraud and error.
Mr Morse said: " Issuing an audit qualification is a serious matter, and the fact that similar qualifications have been in place for such a long period of time does not lessen that seriousness.
"I consider that the overall value of fraud and error in benefit expenditure remains unacceptably high, and the qualification of my audit opinion reflects that.
"We recognise the challenges involved in reducing fraud and error. We are working with the department to develop our approach to evaluating the adequacy of its response to fraud and error in benefit expenditure."
A DWP spokesman said: "We are absolutely committed to doing all we can to reduce the level of fraud and error in the benefit system. The fact is fraud and error is now proportionately lower than before the start of the Parliament and we are recovering more money than ever.
"As part of our long-term plan, we have created a joined up Fraud and Error Service, and will be bringing local authority and HMRC investigators into DWP, and have announced tougher new measures against those who cheat the system.
"Universal Credit is expected to reduce losses due to fraud and error by £1 billion in the next five years when it's fully in place. This modern, simpler and easier-to-administer benefit is running successfully and we are continuing to work closely with local authorities to ensure its continued safe and secure rollout."