Some £940 million in benefits were overpaid last year, according to official figures.
The largest proportion - £400 million - was due to civil servant error, while fraud accounted for £310 million.
The Tories blamed the size of the sum on "chaos and mismanagement" at the Department for Work and Pensions.
Overpayments were equivalent to 5.1% of benefit expenditure in the 12 months to September 2005, with official error blamed for 2.2% and customer error for 1.2%, according to the DWP figures.
Income support overpayments totalled £500 million, while jobseeker's allowance added £120 million, and pension credit £320 million.
Shadow Work and Pensions Secretary Philip Hammond said: "This huge sum in overpayments is further evidence of the chaos and mismanagement at the DWP and a damning indictment of Gordon Brown's complex system of means-tested benefits.
"We need a simpler, fairer benefits system which can be easily understood by the people who need help the most. The current benefits infrastructure is collapsing under the weight of its own complexity."
