An advert claiming a third runway at Heathrow will not make the airport dirtier or noisier has been ruled "misleading" by the Advertising Standards Authority.
The ASA also decided that it was misleading for the advert to claim that the runway would not go ahead "unless local air quality meets stringent EU standards on concentrations of nitrogen dioxide (NO2)".
The advert had been placed in regional papers by pro-Heathrow expansion group, Future Heathrow, and was published in the period before the Government's January 2009 decision to approve a third runway at the west London airport.
The ASA said an MP, who the authority did not name, had challenged the noise, dirt and environmental claims made in the advert.
The ASA said that airport operator BAA, responding to the complaint on behalf of Future Heathrow, had said the noise and dirt claim was a "statement of fact and that readers would be right to confidently understand that a third runway would definitely not create more noise or air pollution than Heathrow at present".
BAA had also defended the NO2 claim. The ASA said today that it found both claims misleading.
The authority went on: "We noted Future Heathrow and BAA firmly believed that the noise and air limits would not be breached, but considered that the evidence we had seen was not sufficient to justify an absolute claim that noise and pollution would not increase following the construction of a third runway.
"We concluded that the claim 'a third runway won't make Heathrow any noisier or dirtier' was likely to mislead."
The ASA said the advert must not appear again in its current form.