The Advertising Standards Authority (ASA) has continued its crackdown against companies that overstate their environmental credentials, upholding a complaint against an advert from oil giant Shell that accused it of making misleading green claims.

The advert, which ran earlier this year, showed an oil refinery emitting flowers instead of smoke and claimed Shell used "waste CO2 to grow flowers and waste sulphur to make super strong concrete".

Friends of the Earth complained to the advertising watchdog that the ad was misleading on the grounds that Shell captures less than 0.5 per cent of its direct CO2 emissions to help grow flowers and uses only a fraction of its waste sulphur to make super-strong concrete.

The ASA upheld the complaint, ruling that it was misleading because readers were likely to believe that Shell was using all, or at least the majority of its waste CO2 and sulphur in the manner described.

A spokeswoman for Shell said that the company accepted the decision, but defended the advert claiming it was intended as a "striking and creative way of drawing attention to the issue of waste disposal".

The UK is rapidly emerging as one of the most demanding advertising regimes in the world with regards to the promotion of green credentials.

Last month the watchdog ruled against npower for using outdated figures for calculating the carbon savings gained from a new wind farm, while earlier this year it issued a comprehensive check list for green advertisers warning against using dubious facts, relying on "pseudo-science" and making sweeping environmental claims.

Speaking at the launch of the checklist, ASA director general Christopher Graham warned that the organisation would be cracking down on firms that overstate their environmental credentials, insisting that "the ASA needs to see robust evidence to back up any eco-friendly claims".

However, Friends of the Earth's corporate campaigner Hannah Griffiths said that the ASA needed to go further still, arguing that while the Shell ruling was welcome it was "a shame that the ASA does not have more teeth and that Shell wasn't fined for its deceptions".