Britain's most controversial new road, the Newbury bypass, has led to an increase in traffic deaths and other serious accidents despite government predictions that it would save lives, a new report revealed today.

The study, published by the Campaign to Protect Rural England (CPRE), pointed out that serious accidents have increased by 20% since the £105m scheme was completed in 1998.

In the mid 1990s, plans for the A34 bypass prompted high-profile protests and concern about its environmental impact on an area of outstanding natural beauty outside the Berkshire town.

At the time the Department for Transport defended the 8-mile bypass by predicting that it would result in a 47% reduction in road deaths through Newbury. But the CPRE's new analysis of the government own evaluation of the scheme, which was published in July, highlighted the fact that deaths and serious injuries on the A34 corridor rose from 30 in the four years before the bypass was built to 45 in the four years afterwards.

Road deaths alone have risen from six to 10 in the past five years, the report pointed out.

"This is a very heavy price to pay for saving between four and 11 minutes in journey times," said Shaun Spiers, the chief executive of the CPRE.

The CPRE also accused the government's Highways Agency of bias in its evaluation of the road.

Announcing its findings in a press release, the CPRE said: "The consultants hired by the Highways Agency [to conduct the evaluation] emphasised favourable figures and neglected damaging ones in order to portray the new road in the best possible light.

"Evaluations need to be more objective and more independent," it continued.

The CPRE report, written by transport consultants Ian Taylor, John Elliott, Lynn Sloman and Lilli Matson, also found that the Highways Agency had underestimated the number of vehicles that would use the bypass, and overestimated how much it would cut congestion in Newbury.

The agency predicted that up to 36,000 vehicles would use the bypass by 2010, but last year daily traffic had already reached 45,900.

The CPRE said that that congestion during the morning rush hour in Newbury had reached the same level as it was before the bypass had opened.

Mr Spiers said: "We strongly opposed the bypass because we knew it would generate extra traffic and cause increased sprawl. This belatedly published official evaluation [the Highways Agency's findings] shows [that the bypass] has done both of those, but it has also proved more dangerous."

The Highways Agency defended the bypass and its evaluation of the scheme.

In a statement it said: "More traffic than envisaged using these roads means there is less traffic on local roads, which as a result are safer for people using them. Less traffic now drives through Newbury and other towns on these roads because of our road schemes."

It also pointed out that it has changed the way it appraises new schemes since the bypass was built.

In July a conservation group revealed that a rare snail species that almost halted the construction of the bypass had become extinct in the area. Campaigners had tried to use the presence of the protected Desmoulin's whorl snail in the area to halt the project.