"I’ve directed TfL [Transport for London] 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," Khan said in a statement.
Khan reaffirmed his previous resolve to not let "hundreds of thousands suffer from the additional noise and air pollution that a third runway would cause."
The UK Transport Department announced last month its support for a new full-length runway at Heathrow.
The government's original plans included possible construction of one more runway in Heathrow or in the country's second busiest airport of Gatwick, as well as the extension of the existing runway in the biggest UK airport.
