The car company BMW has been censured by the UK’s advertising watchdog for claiming an electric car equipped with a small petrol engine was “clean” and “zero emissions”, in a ruling that could have a knock-on effect on other electric car advertising.

The advertising was published in the form of a Facebook post that used testimonials from real customers to extol the virtues of the BMW i3. That model is unusual among electric vehicles, as in addition to the electric drive, it also has a small petrol engine. However, unlike “hybrid” cars, which have a petrol-driven engine that can take over from the electric system when it runs out of charge, on longer journeys or at higher speeds, the i3’s petrol engine is only used to maintain the charge on the electric drive.

In the Facebook ad the car was described as “a clean car and helps to give back to the environment”. A complainant suggested that the presence of a petrol engine meant this statement was incorrect.

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BMW argued that the nature of the petrol engine, in maintaining charge for the electric engine rather than taking over from it when needed, meant its description – which came from a user – was accurate. The Advertising Standards Authority (ASA) took a different view, saying that the petrol engine had an emissions value that could be quantified, and has ordered the advertisement to be withdrawn.

The ASA said: “We told BMW to ensure that in future they made clear their environmental claims related to the all-electric vehicles only and not to make environmental claims about their products unless they held sufficient substantiation.”

BMW did not respond to the Guardian’s request for comment. The Society of Motor Manufacturers and Traders said it did not comment on individual cases, and declined to comment on the broader issues of how “clean” cars can be defined.

The question of what constitutes “clean” driving is a vexed one. Electric vehicles produce no emissions at the point of use, but the power they consume does produce greenhouse gases if it comes from fossil fuels, such as coal or gas-fired power stations. This means even the cleanest electric vehicle can only have as little impact on the environment as the electricity grid from which it is powered.