The cost of widening the M25 motorway has increased by a quarter because of failings by the Highways Agency, a report has said.
The project covers 22 miles (35km) between junction 16 and junction 23.
But an 18-month delay meant the contract was let in May 2009 at the height of the credit crisis, a National Audit Office (NAO) report said.
This increased the cost by £660m to £3.4bn, the report said. The Highways Agency said it would respond later.
The delay occurred because the Highways Agency was testing whether using the hard shoulder at peak times could negate the need for motorway widening.
This testing took longer than expected, postponing the start of procurement for the road works.
NAO head Amyas Morse said: "The Highways Agency's project to widen the M25 could have achieved a materially better value-for-money outcome.
"This was partly because the slowness with which the project was taken forward subjected it to the cost effects of the credit crisis.
"The agency should have adopted a more agile approach to procurement."
Roads minister Mike Penning said: "This government is driven by the need to get value for money for taxpayers so I welcome this report.
"I am determined to learn the lessons of the report and we will act on its recommendations."
Junction 16 is where the motorway joins the M40 while junction 23 is the A1(M) interchange.
A spokesman for the Highways Agency said it would respond to the NAO report at a forthcoming meeting of the Public Accounts Committee.