The Scottish Executive has run into criticism for its decision to scrap tolls on the Forth and Tay bridges after an independent report warned the move would increase congestion and harm the environment.

The toll impact study, which was commissioned by the previous administration, claimed their removal would see the number of vehicles using the Forth road bridge rise by around 10% and traffic on the Tay bridge go up by as much as 40%.

"The removal of tolls would lead to increased global and local traffic emissions," according to the report, carried out by consultants Steer Davies Gleave, who concluded that the charges should be retained.

The publication of the report yesterday came as official figures revealed traffic volume and the number of cars on Scotland's roads are at their highest ever levels.

Duncan McLaren, chief executive of Friends of the Earth Scotland, said the toll report showed the executive had got its priorities wrong. He said: "This proves that it's going to be a very big challenge for the SNP to marry their transport agenda and their promises on climate change.

"It's disappointing that we've seen them keep the promise to scrap bridge tolls, but downgrade their commitment to reducing carbon emissions by 3% every year."

As an alternative to scrapping the tolls, Friends of the Earth have proposed increasing the charges at peak times as a way of controlling traffic volume. That policy is supported by the Greens, whose transport spokesman, Patrick Harvie MSP, yesterday described the executive's policy as "ridiculous".

"Congestion causes economic damage as well as environmental problems, and pursuing policies which make it worse is simply absurd," he said.

Scrapping the tolls was an SNP manifesto commitment agreed by parliament shortly after the party came to power in May.

John Swinney, Secretary for Finance and Sustainable Growth, yesterday insisted it was the right decision. He said: "We will continue to invest in initiatives which reduce congestion, such as improved park-and-ride facilities, and improved rail, bus and cycle links."

Harry Wills, acting chief executive of Dundee and Angus Chamber of Commerce, backed scrapping tolls, describing them as a "tax on business".

An executive report on transport trends, published yesterday, showed the number of cars on Scotland's roads was almost 2.6 million, 2% up on the year before and the highest number ever. The amount of vehicles on Scotland's motorways went up by 5%.

Elsewhere, the statistics revealed the number of air passengers increased by 3% in the past 12 months, 85% over the past decade, while the number of train commuters also went up by 3%.