Firms confused over how to use green terms in their advertising and marketing campaigns without falling foul of customers or regulators will receive new guidance later this year.

The Committees of Advertising Practice (CAP), which are responsible for the UK's advertising codes, are currently working on a series of guidelines for advertisers in the wake of a series of rulings from the Advertising Standards Authority (ASA) that have criticised firms for making misleading claims about their green credentials.

Advertisers have repeatedly criticised the watchdog's rulings, insisting their green claims are being made in good faith and that any confusion is the result of a lack of clear definitions for new environmental terms, such as zero carbon or offsetting, rather than a deliberate attempt to mislead customers.

A spokesman for the ASA said that CAP was working to resolve the problem and would release a series of guidance notes over the course of the year that will set out clear guidelines on what is green claims are acceptable and what is understood by various environmental terms. "CAP is looking at all the [ASA] rulings recently and clarifying our position," he added. "For example, there has been a lot of confusion around what you can say about carbon offsetting schemes and the guides will look to address that."

Marina Palomba, legal director at the Institute of Practitioners in Advertising (IPA) welcomed the move, adding that the IPA would support a strict set of guidelines. "This is a very difficult and complex area and one where consumers can easily be misled so the IPA supports a strict interpretation of any environmental or green claims," she said.

The ASA said the notes will be distributed to industry and would be made freely available on the CAP website. In the meantime, the watchdog is advising firms to check with CAP's Copy Advice team for clarification on how the ASA is likely to assess a green ad before publication.

The guidance can not come soon enough for many firms that have become increasingly frustrated by the lack of clarity over which green claims are deemed acceptable.

British Gas today reacted angrily to an ASA ruling banning a TV and newspaper advert for its Zero Carbon energy tariff. The ASA ruled the adverts were misleading as the TV ad claimed the energy was zero carbon when the tariff relied on offsetting schemes and the print advert claimed it was "the greenest domestic energy tariff", despite there being no standard methodology for measuring tariffs' greenness.

A spokesman for British Gas said the company "fundamentally disagreed" with the ruling, arguing that it had stated in the TV advert that the tariff used carbon offsetting schemes and that the print advert made it clear the claim was based on independent data from the energywatch website.

"It is disappointing that the ASA has underestimated how commonly used and understood the term 'zero carbon' is, especially considering we consulted the Broadcast Advertising Clearance Centre (BACC) when we produced the [TV] advert, " he said.