The comment comes after the European Commission published its plans for Euro V emission levels.

These would cut particulates from new diesel engines by a factor of five, make permitted NOx limits much tougher for petrol and diesel models and raise the durability requirements for emission control systems like catalytic converters and particulate traps.

Maximum particulate levels under the latest Euro IV standards are already 90 per cent tougher than they were 15 years ago.

Under the new proposals, limits would be cut further, from 25 to five milligrams per kilometre. These levels mean new diesel vans will need particulate traps, increasing their prices. This could halt the diesel car sales boom and even see more vans switch to petrol. Much of the 10 per cent improvement in average CO2 emissions comes from new cleaner diesel cars and vans so SMMT says the latest plans should be treated with caution.