The Labour politician said that Transport for London (TfL) was preparing to join four Conservative councils in a combined High Court case against expansion of the west London terminal.
Khan, who strongly backed the building of a second runway at Gatwick, told the London Assembly this week that he "would not stand by" and see residents "suffer increased air and noise pollution".
“I promised I wouldn’t just stand by and see hundreds of thousands suffer from the additional noise and air pollution a third runway would cause,” he said.
“That’s why I’ve directed TfL to provide their expert advice and assistance to support Hillingdon, Richmond, Wandsworth and Windsor and Maidenhead councils, and Greenpeace, as they prepare for a joint legal challenge, and why I will be ready for us to play an active role in the action if required.”
He added that Heathrow expansion would mean the “intolerable” prospect of an extra 200,000 Londoners being exposed to unacceptable levels of noise every day, leading to health problems related to stress and sleep disturbance, the London Evening Standard reports.
Writing in The Times, Khan added: “We always knew that if the government opted for Heathrow, this process would drag on and be subject to delays and dithering.
“The elephant in the room is whether it is actually possible to deliver it at all, considering the legal and political obstacles in Heathrow’s way.
“There is a better choice. The evidence is clear that an expanded Gatwick would not only provide a substantial boost to our economy but could be delivered without causing the huge environmental damage that Heathrow will inflict.
“As we move forward, the government should be in no doubt that I stand ready to play an active role in this process. Ministers will not be able to ignore Londoners’ democratic views without a fight."
