The recommendations, which were endorsed last week in a vote on a European Parliament Plenary report, have been criticised by the Federation of European Direct and Interactive Marketing, which said the warnings "would be ignored" in the same way smokers ignored warnings on packets of cigarettes.
The proposed warnings form part of the European Commission's pledge to curb CO2 emissions, and could force the automotive industry to carry health warnings on all direct marketing and promotional literature activity.
Under the proposed guidelines, marketers in the automotive industry would have to carry details that occupy 20% of the total space on all communications about a car's fuel consumption ratio and environmental costs and impacts.
Fedma said the proposed rules could impact on the amount of direct marketing produced by car manufacturers and questioned how the strategy could be enforced online. The 20% total space figure would mean car brands would have to carry four pages of CO2 emission warnings for every 20-page brochure.
Alastair Tempest, director general of Fedma, said: "Fedma believes that this proposal would have the effect of reducing the size and amount of direct marketing for cars and other vehicles -- and how exactly it would work for websites is very unclear.
"It is very probable that in the medium to long term, consumers would simply ignore this information, and thus the measure would be counter productive. This has happened with the large health warnings on tobacco promotion and packaging. The consumer needs the details of CO2 emissions, but this is not the effective way to provide it."
Fedma said there is already an EU directive on environmental labelling outlining the legal requirements for advertisers, marketers and car brands and this should form the basis of industry guidelines.