Speaking at Mayor's Question Time yesterday, 16 November, Mr Khan said he was "ready to play an active role" in joint legal action against the £18.6 billion development.
Critics argue the project would increase noise pollution for some 200,000 Londoners as well as leading to a rise in NO2 emissions which are already above legal limits.
Transport for London (TfL) is expected to named in legal action as an interested party in a move which has been welcomed by campaigners.
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Mayor Khan said: "I promised I wouldn't just stand-by and see hundreds of thousands suffer from the additional noise and air pollution that a third runway would cause. These decisions show that I'm ready to help ensure that this misery isn't inflicted on Londoners.
"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."
Leader of Richmond Council, Lord True, added: "TfL are the best informed organisation on London transport links and know that the transport connections required for a third runway are simply undeliverable at the cost suggested by Heathrow PR men or without further air quality and noise pollution for Londoners."
(LM/CD)
