The European Federation for Transport and Environment (T&E) says it is because average weight rose by 1.4%.

BMW managed to reduce its average emissions, but that was offset by rises from DaimlerChrysler and Volkswagen.

French, Italian and Japanese firms all managed to cut their average emissions over the period.

"It is ironic that the country that did so much to get a European consensus on new climate targets earlier this year is also home to the carmakers that are holding back progress on one of the most important ways of achieving them," said Jos Dings, director of T&E.

"The failure to cut the weight of cars is one of the principal reasons why CO2 emissions and fuel consumption are not going down," he added.

France's PSA Peugeot Citroen sold the lowest carbon dioxide emitting cars in 2006.

Last month the European Parliament proposed extending an EU deadline for car makers to develop more eco-friendly vehicles to 2015 from 2012.