British Airways has been reprimanded for attempting to manipulate a government consultation on the expansion of Heathrow by making false claims about the environmental impact.

The Advertising Standards Authority has written to BA ordering it to withdraw a claim, in an e-mail to customers by Willie Walsh, the chief executive, that a new runway would reduce carbon dioxide emissions.

Mr Walsh wrote to tens of thousands of members of the airline’s Executive Club in November asking them to support the construction of a third runway. He sent the e-mail the day after the Government announced a three-month public consultation on expanding Heathrow. Mr Walsh claimed that expansion would reduce carbon dioxide emissions by 330,000 tonnes a year because aircraft would no longer have to waste fuel queueing for take-off or circling while waiting to land.

His e-mail implied that the claim about CO2 reduction had been endorsed by the Government. But he failed to make clear that, according to the Department for Transport’s consultation document, the new runway would raise overall CO2 emissions by 2.6 million tonnes a year by allowing an extra 220,000 flights.

The authority has asked BA to confirm by January 9 that it will not repeat the claim. In a letter to a BA customer who complained about the e-mail, the authority wrote: “We believe you have a valid point and have instructed BA to change their ad. We have asked them for a written assurance that any future marketing communications about the expansion of Heathrow airport will not suggest an overall decrease in CO2 emissions.”

BA refused to say last night whether it would comply with the authority’s letter. It also refused to say how many passengers had been sent the e-mail or whether the airline would now send them a clarification. In a statement, the airline said:“By the time a third runway opened, emissions trading – for which British Airways has long campaigned – would ensure that every extra tonne of carbon dioxide from aviation growth above 2005 levels would be matched by a tonne saved elsewhere.”

Times recommends

• Prince Philip attacks big families
• Take a dip in Dubai-on-Thames
• Eco Worrier: How can I persuade my husband to use a push mower?