The "green levy" on motorists announced in Alistair Darling's first Budget will double car tax revenue to £4 billion but reduce vehicle emissions by less than one per cent, Treasury figures have showed.

The Chancellor announced a significant increase in car tax in March.

This will result in the owners of family cars, estates and people carriers paying hundreds of pounds a year more to use the roads.

Mr Darling claimed that the duty increase was designed to encourage motorists to switch to greener cars and to reduce the environmental impact of driving.

However, the Telegraph has seen Treasury projections which disclose that while the amount raised from car tax will more than double - from £1.9 billion to £4.4 billion by 2010 - carbon dioxide emissions from motoring are expected to drop by less than one per cent.

Justine Greening, a shadow Treasury minister who obtained the figures, said last night: "This is a massive tax hike which will have virtually no impact on the environment.

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

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