Diesel-engined Jeeps and Renaults can produce nine times the legal limit for nitrogen oxide emissions, according to research from Which?.
The consumer organisation tested 16 diesel-powered Renaults, and found that all of them were emitting up to nine times more harmful nitrogen oxide (NOx) than the Euro 6 limit of 0.72g/km.
Vehicles from American 4x4 brand Jeep were even worse offenders, with older Euro 5 cars emitting an average of 1.735g/km – nine-and-a-half times their prescribed limit of 0.73g/km.
Nissan, one of Renault's partner brands, also performed badly, with the six vehicles tested by Which? emitting an average of 0.8g/km, which is again above the legal maximum.
In response to the news, Renault said: "All Groupe Renault (the brands Renault and Dacia) vehicles are, and always have been, homologated in accordance with the laws and regulations for all the countries in which they are sold. They all conform to the current standards.
"However, Groupe Renault has been aware that there remains significant potential for improvement regarding the release of NOx in real-use conditions.
Richard Headland, Which? magazine editor, said: "While our tests show that some car manufacturers are making progress on reducing the amount of toxic emissions from their models, many have a long way to go in cleaning up their act.
"We hope that the improved official tests being brought in later this year will more clearly name and shame those manufacturers that are failing to meet their obligation to lower emissions."
AOL Cars contacted Jeep for comment but has not yet received a response.