A clampdown on fake and exaggerated “green” claims has been launched by Britain’s advertising watchdog after a flurry of complaints about unsubstantiated environmental boasts by some of the world’s best-known companies.

After upholding complaints about companies including Toyota, Volkswagen and Scottish & Southern Energy, the Advertising Standards Authority (ASA) has told businesses that they must not misrepresent or overstate their claims to being eco-friendly.

In the rush to win over environmentally conscious customers, it says, companies are too easily associating their products with “buzz” phrases such as “carbon offsetting” and “carbon neutral” without providing evidence to back up their claims.

The watchdog is reminding companies that any such claims must be supported by proof. Companies should also not present claims as being universally accepted if the science is, as yet, inconclusive, it says.

In addition, it is telling businesses to avoid sweeping statements about being “environmentally friendly” if there is no way that they can prove it.

As eco-friendliness becomes universally accepted as worthy cause, companies are jostling to prove their green credentials and tap into the lucrative market for associated products.

A recent report from the Soil Association found that last year organic food and drink sales in the UK approached the £2 billion mark for the first time.

Retail sales of organic products were worth an estimated £1.9 million, up 22 per cent on the previous year. Britain’s organic market is now the third-largest in Europe, after Germany and Italy.

Marks & Spencer recently announced plans to spend £200 million over the next five years on going green. The move followed the group’s Behind the Label campaign, highlighting its environmental and ethical business practices, which was one of its most successful advertising schemes.

However, scrutiny of some companies’ claims has found them wanting: Since October 2006 the ASA has upheld 17 complaints about advertisers who have misled consumers by making unproven claims.

Christopher Graham, the director-general of the ASA, said: “Companies are seeking to tap into greater awareness and concern about green issues, but they need to be careful they are not getting it wrong.

“If a company makes a claim that is not [fair], the ASA will check it out and find you out.”

Companies that have been criticised by the authority include Toyota. The ASA upheld a complaint that a television advertisement for its Prius vehicle misleadingly exaggerated the environmental benefits of the car.

The complaint centred around the advert’s claim that the Prius “emits up to one tonne less CO2 per year” than equivalent family cars with a diesel engine. The ASA said it did not consider that Toyota’s evidence demonstrated that it emitted one tonne less than equivalent vehicles with diesel engines.

Scottish & Southern Energy has also come under fire from the watchdog over claims in a leaflet that it would plant trees to balance the CO2 produced by consumers’ gas heating and household waste.

The ASA said that the energy group could not provide evidence to show the amount of CO2 that would be absorbed by the trees planted - hence it was not possible to determine whether the trees would balance the CO2 produced by the gas heating and household waste of the average British household.

Times Recommends

• Toyota recalls 437,000 cars including Lexus
• Myners says bonuses are bleeding pension funds
• Japanese company faked safety reports on seats