Campaigners have threatened legal action over the "manifestly unfair and unlawful" consultation into air pollution taken by the Airports Commission last month.
The Teddington Action Group, which aims to prevent expansion at London Heathrow, said the last-minute, three-week air quality consultation (see news, May 11) was "unacceptable".
It claims there was a "lack of proper engagement" and has now taken the first step towards launching a judicial review.
The group has also demanded that chairman Sir Howard Davies step down due to "presumed bias" because of his appointment as Royal Bank of Scotland chairman.
The move is the first legal challenge to the commission's work and comes just weeks before it backs either a second runway at Gatwick, a third at Heathrow, or extending one of the two existing runways at LHR.
Teddington Action Group spokesman Paul McGuinness said: "The lack of proper engagement by the commission in relation to the latest air quality consultation is unacceptable and local people should be consulted in a meaningful way on an issue that directly impacts their health and wellbeing."
Last month, Gatwick CEO Stewart Wingate warned that expansion at Heathrow would be "unlawful" as it would further breach air pollution limits (see news, May 29).