The European Union is currently looking at plans to close the loophole which enables heavy four-by-four vehicles to avoid emissions targets for new cars.

At present, passenger vehicles with a mass of over 2,500kg are able to comply to less ambitious standards than lighter cars.

However, the EU is now preparing to change that position, after launching the latest Euro 5 limits for consultation, which will see larger vehicles having to comply to the same limits as other cars.

That will mean the vehicles will have to have a manufacturer's guarantee of longer life for catalytic converters and other emissions-reducing equipment, as well as adhering to strict limits on emissions.

The Euro 5 standard could come into force as early as summer 2008, and at present it proposes particulate emissions for diesel vehicles to be cut by 80 per cent , while petrol vehicles would see a reduction of 25 per cent in nitrogen oxide and hydrocarbon limits.

Although admitting that the aims are "ambitious", European Commission (EC) vice president Günter Verheugen insisted: "This is the only road to future success."

He added that the plans are "realistic", and urged the stakeholders to back the proposals during the consultation process.