The advertising watchdog has banned Renault from repeating an advert in the UK which claimed it was reducing CO2 emissions but quoted French figures.
The advert said the electric Renault Fluence Z.E. would cut emissions by 90% compared with a diesel model.
But the Advertising Standards Authority upheld a complaint that the French electricity generation mix emitted less CO2 than that produced in the UK.
The advert's small print said the calculation was based on the "French average electric mix" - prompting a reader to complain to the ASA.
Renault insisted its figures were not misleading because it expected to sell significantly more vehicles in France than in the UK.
It also said there were a number of electricity providers in the UK which offered carbon-free or reduced-carbon tariffs, meaning consumers could buy energy with CO2 emissions similar to the French average mix.
But the ASA ruled that readers were unlikely to understand the difference between electricity generating mixes in France and the UK and how this would affect CO2 savings.