According to reports in The Times, the ASA has written to British Airways (BA) asking it to confirm by this Wednesday that it will not repeat claims that the proposed third runway would reduce carbon dioxide emissions by 330,000 tonnes a year by limiting the need for planes to circle the airport while waiting to land.

The claim was made in an email from chief executive Willie Walsh to members of the airline's Executive Club in November, which urged them to support the construction of a third runway.

However, the ASA received a complaint about the email, arguing that it implied that the third runway would deliver a net reduction in carbon emissions. According to government figures, the third runway would lead to an overall increase in emissions of 2.6m tonnes a year by enabling 220,000 extra flights. The ASA reportedly supported the complainants view and informed him that it had asked BA for "a written assurance that any future marketing communications about the expansion of Heathrow airport will not suggest an overall decrease in carbon dioxide emissions".

A spokesman for the ASA said that the investigation was ongoing and a formal decision would be reached within the next few weeks. He added that the number of complaints the organisation has received over the last year concerning companies' environmental marketing claims had "snowballed", as consumers become ever more wary of businesses' green ads.