Oil giant ExxonMobil is considering an appeal after the Advertising Standards Authority (ASA) today slapped a ban on its high profile TV adverts suggesting that liquefied natural gas represents "one of the world's cleanest fuel".

In a move that could result in a significant test case on the language firms can use when promoting their green credentials, Nick Thomas, corporate affairs director at Exxon subsidiary Esso UK, said that the company maintained the ad " was accurate and truthful" and was considering an appeal.

Upholding four complaints against the ad, the ASA said that it was unintentionally misleading as viewers were likely to measure the claim that liquefied natural gas represents one of the world's cleanest fuels against the other forms of energy mentioned in the advert, including nuclear, wind and solar, and not other forms of fossil fuel as Exxon intended.

It also ruled that as the ad started with an Exxon employee claiming that "I think one of the biggest challenges that the world is facing today is to develop all the energy we need in an environmentally friendly way", viewers would interpret liquefied natural gas as an "environmentally friendly" fuel when in fact it has significant carbon emissions.

Thomas said that ExxonMobil was "very disappointed" with the decision adding that both Clearcast, the organisation responsible for reviewing television advertisements before transmission, and the ASA's own Investigations Executive had felt the ad was justified and the complaints should not be upheld.

The company now has 21 days to appeal the decision.

A spokeswoman for the ASA said that were ExxonMobil to appeal the decision would be considered by an independent adjudicator who could recommend that the ASA Council take a second look at the advert. If the adjudicator decided not to refer the case for further consideration Exxon could then resort to the courts to appeal the decision, although the spokeswoman said that such instances were very rare.

She added that despite the Exxon case there was evidence that firms were improving the clarity and accuracy of their green ads as concerns over greenwashing grow. "Last year we had complaints about 410 different ads, and to date this year we've only had complaints over 200 ads," she said. "The peak in the number of complaints has passed and that is not because viewers are less concerned but because the ads are getting better."

She added that firms should continue to be careful not "distil lots of scientific information into a sound bite that could be misconstrued".