The environmental benefits of the levies introduced in Alistair Darling's first Budget have been called into question by the Treasury's own figures.

Among the measures set out by the Chancellor last month was re-working road duty to punish cars with higher CO2 emissions.

Gas-guzzling new cars will also be subject to a special higher rate for the first year - described as a "showroom tax".

However, Treasury projections obtained by the Tories using Parliamentary questions indicate that the reduction in environmental damage will be minimal - despite tax revenue increasing sharply.

The Exchequer's annual income from car duty is set to rise from £1.9bn to £4.4bn by 2010, but emissions from motoring are expected to drop by less than 1%, according to the forecasts.

Shadow Treasury minister Justine Greening said: "This is a massive tax hike which will have virtually no impact on the environment.

"Despite their claims, the Government doesn't expect this move to change behaviour at all -- it is just another eco stealth tax of the worst kind."