The Highways Agency's slow response to the widening of the M25 - Britain's busiest motorway - has led to a 24% increase in costs, according to a report by National Audit Office (NAO).
The £660 million increase, which has seen the net present cost reach £3.4 billion, has been attributed to an 18-month delay in preparing and finalising the widening procurement for the M25, the Government spending watchdog said.
TheNAO report said the Government had revealed intentions to reduce congestion on the M25 in 2003.
During that period, the Highways Agency was examining potentially cheaper alternatives, such as allowing drivers to use motorway hard shoulders at peak times - known as hard shoulder running (HSR).
According to the NAO, a conventionally procured HSR solution for the M25 could save between £400 million and £1.1 billion.
The report said: "The slow progress on testing HSR and the agency's commitment to widening meant that the agency limited its options.
"As a result, it did not give itself the opportunity to secure a better price for the taxpayer. The agency is now using HSR on a number of major roads."
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