An advert for one of Britain's largest renewable energy suppliers has been banned for claiming the electricity it supplies emits no carbon dioxide.
Good Energy's website said in September: "An average unit of electricity in the UK (a kilowatt hour or kWh) results in 360g of carbon dioxide (CO2) emissions and 0.007g of radioactive waste.
"But the electricity we supply contains 0g of CO2 and no radioactive waste. This will never change."
A reader, who believed that Good Energy used biomass energy, which they understood produced more CO2 than coal when burnt, complained that the company's claim was misleading.
Good Energy, which supplies around 200,000 customers, gave the Advertising Standards Authority (ASA) information from the Department for Business, Energy and Industrial Strategy (BEIS) saying biomass fuels were considered a form of renewable energy, and that renewable energy sources produced 0g/kWh of CO2 compared with coal which produced 925g/kWh.
The ASA said consumers would understand Good Energy's claim to mean that no CO2 was emitted in generating the electricity and supplying it to consumers, regardless of whether the source was a form of renewable energy.
It noted that advertising rules required environmental claims to be based on the full life cycle of the product, which in Good Energy's case included import or export, transport of the raw material to processing plants, crushing and processing to produce biofuel and transport of the biofuel.
The ASA said: "We considered even if those emissions were offset, this did not mean no CO2 was emitted.
"Because we considered the claim would be interpreted to relate to the amount of CO2 emitted throughout the full life cycle of all sources of Good Energy's electricity, we concluded the claim that the energy supplied by Good Energy contained 0g of CO2 had not been substantiated and therefore was misleading."
It ruled that the ad must not appear again, adding: "We told Good Energy not to claim that the electricity they supplied contained 0g of CO2 unless they held evidence that no CO2 was emitted throughout the entire life cycle of the product."
A Good Energy spokesman said: "We always aim to be as fair and straightforward as possible in our communications, and have historically reported on CO2 emissions based on guidance from the Greenhouse Gas Protocol and UN in respect of carbon reporting.
"However, it can sometimes be difficult to manage simplicity against detail so we welcome the ASA's guidance regarding how to give consumers greater clarity, particularly in an industry that isn't always simple.
"We will take this ruling as an opportunity to extend our efforts at transparency further and provide wider clarification and look forward to seeing how it is applied not just in renewables, but across the energy sector."