Proposals for tougher EU emission limits for petrol and diesel engines could raise new car and van prices, according to the Society of Motor Manufacturers and Traders, after the European Commission published plans for Euro 5 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 4 standards are already 90% 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 cars and vans will need particulate traps, increasing their prices.

The SMMT said this could make diesel less attractive to potential buyers and halt the diesel car sales boom seen over the last six years. Much of the 10% improvement in average new car CO2 emissions comes from new cleaner diesel cars and vans so SMMT believes the latest proposals should be treated with caution.

Chief executive Christopher Macgowan said: "The industry is committed to cutting all emissions from new cars and vans and we have come a long way in the last 15 years. However, we need to be clear that the new proposals could mean additional costs to vehicle makers and their customers.

"Additionally, in focusing on one type of exhaust emission, there is the potential to ignore challenges this may create in other areas. If not handled carefully, Euro 5 proposals could stall progress in cutting average carbon dioxide from new cars."

The UK market for new diesel cars rose from 303,925 units to 835,334 between 1999 and 2004. Diesel cars now account for one in three of every new car sold. Fuel consumption and corresponding carbon dioxide emissions are around 20 to 30% lower for diesel models than for petrol equivalents. Over 99% of all new vans sold are diesel powered.