The TV ad showed a cartoon blue gas flame falling onto the text "CO2" with a voiceover that said: "Of all the major suppliers in Britain, our CO2 emissions for electricity are already the lowest".
The blue flame landed in the text "CO2" and squashed the "O", while the voiceover said: "and our new dual fuel package is our greenest. With zero carbon". The blue flame became coloured green and pushed away the "C" and the "2" as small print at the bottom of the screen read: "Relates to offsetting schemes".
Fourteen viewers argued that the claim "zero carbon" implied the fuel used was carbon-free and did not produce carbon dioxide but that this was untrue because as it is a fossil fuel, carbon dioxide is always a by-product of its use.
British Gas said that "zero carbon" was the name of its domestic dual tariff for gas and electricity, which resulted in a net impact of zero for carbon emissions for each customer.
The company explained that the tariff was backed by 100% renewable electricity and that it offset the carbon emissions associated with the gas and electricity used by its customers.
British Gas said that the footnote, "Relates to offsetting schemes", made it clear that carbon emissions were expected, but that they would be dealt with through an offsetting scheme. The supplier said that qualitative consumer research for the ad had indicated that it delivered this message to viewers.
However, the Advertising Standards Authority ruled that viewers were likely to infer from the claim that gas supplied by British Gas was carbon free. The ASA argued that the footnote "Relates to offsetting schemes" contradicted the overall impression of the ad.
The watchdog therefore concluded that the ad was misleading, upheld the complaints and disallowed it from future use.
After a separate complaint, British Gas had to pull a national newspaper ad about its zero carbon tariff after rival Scottish & Southern Energy complained about the claims it made, including one that said British Gas had the greenest domestic energy tariff compared with those on the Energywatch website.
The ASA upheld Scottish & Southern Energy's complaint, which pointed out that the Energywatch website does not evaluate a tariff's"greenness" and therefore British Gas's claim in the ad was false. British Gas was told not to publish the ad again in its current form.