Britain's benefits system is too complex and several billion pounds were lost last year through error and fraud, a spending watchdog has said.
The benefits system has been overhauled many times
A National Audit Office (NAO) report recommends the system be simplified to run more efficiently.
An estimated £2.6 billion was lost over the past year - most of which was due to human error, the report said.
Anti-fraud minister James Plaskitt said progress was being made, but the Tories argue the system is too complicated.
Each month the Department for Work and Pensions (DWP) and its agencies send out some 600 million payments to about 30 million people.
The NAO report says the gradual introduction of new benefits as well as a constant flow of changes have created a tangle of fiendish complexity.
This makes the system difficult to grasp by staff and claimants alike, leaving it vulnerable to error, which accounts for the majority of money lost.
NAO head Sir John Bourn called on the government to come up with a strategy to tackle the problem.
"There is a balance to be struck between a system which is detailed enough to respond to needs and yet straightforward enough to be run efficiently," he said.
Speaking on BBC Radio 4's Today programme, Mr Plaskitt said Sir John had found the DWP was making "good progress".
But, he added: "The complication in the system does contribute to the error. Mistakes can be made both by people applying for the benefits and by our staff administering the benefits."
Mr Plaskitt said fraud was falling and simplification of the system was already under way, with more proposals to be put forward in a consultation paper on welfare reform next year.
Conservative spokesman on work and pensions David Willetts told the BBC that reducing the number of means-tested benefits was the key to reforming the system.
"It is now collapsing under the weight of its own complexity with people getting money they are not entitled to and others not getting help when they need it," he said.
Liberal Democrat work and pensions spokesman David Laws criticised delays in reform of the benefits system, describing it as "one of the great disaster zones of Labour's third term".
In its report, the NAO identified seven key principles that it said the benefits system should take into account.
They include designing new benefits, sharing information and avoiding duplication and using technology to protect claimants from complexity.
The report will be handed to Parliament.