“We're already under a strict code of practice in print media,” said an SMMT spokesman and doubted whether the proposals would ever be incorporated into EU law.

The proposals state that 20 per cent of any ad space, including TV or radio airtime messages, must be dedicated to messages about CO2 emissions levels.

Peter Davies of advertising and PR agency Ward Lovett has spoken out against the plans. He said: “I believe that the environmental information available in brochures, on manufacturers' websites and on the cars on display is sufficient. “While we accept that environmental information is important and should form part of a motorist's buying decision, it is excessive to carry that information in advertising.” He concluded that the proposed warnings would create “just more waffle to wade through”.

MEP Chris Davies, who is behind the scheme, disagreed. “Car manufacturers claim they simply respond to consumer demand, but I believe they use advertising to shape consumer demand,” he said. “It is time to insist that advertisements give car buyers more details of the fuel economy and emissions performance of the vehicles on sale.”

Meanwhile, the SMMT has come out in favour of Britain's proposal to extend car emissions targets to fall between 2020 and 2025, as reported by the Times. “Our position is that any targets set need to be long term,” said a spokesman from the automotive society. “‘Long-term' is the watch word for the industry.”

The later target would give the industry a chance to plan and develop new products in order to fulfil the requirements effectively, the spokesman continued. Transport secretary Ruth Kelly has said she will urge the EU to consider a target of 100 g/km for the average new car between 2020 and 2025. This target is even later than the 2015 goal backed by the European Parliament last month. The EP voted in favour of a 125 g/km target by 2015, rather than the original 120 g/km by 2012 set by the European Commission.