In an announcement made during the Mayor's Question Time in City Hall on Wednesday November 16, the Labour Mayor said he "will fully support" boroughs such as Hillingdon and Richmond as they fight against the government's expansion choice .
Addressing the full London Assembly , he said: "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.
"That’s why I’ve directed Transport for London (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."
Greenpeace has already joined forces with Hillingdon council to take legal action against the proposals.
In the previous Mayor's Question Time, Mr Khan said he wasn't able answer the question on legal action until the decision had been made , but the official announcement was met with delight from many Assembly Members.
Heathrow Airport told getwestlondon: "The Airports Commission’s detailed work represents a solid foundation to support a Government decision for the third runway, and we are confident that any legal challenge would be unsuccessful and would not affect the timeline for delivering a third runway.
"We want to work with the Mayor to ensure that Heathrow expansion helps London thrive as a global hub for talent, tourism and trade."
It has been proposed a the third runway could be built over the M25.
Transport Secretary Chris Grayling previously said it would be a "cheaper and quicker" option rather than building a tunnel for the M25 underneath the new runway.
"I am, of course, very concerned to make sure that, as this runway is built, it doesn't cause massive disruption on the M25, so I think this is a sensible way," Mr Grayling said.
"It is a very gentle hill up which the planes would take off rather than a flat surface.
"It's what happens at very many airports around the world."
According to the airport's timeline, it is unlikely that any new runway capacity would be operational before 2025.
West London has remained firmly divided as the decision to expand at Heathrow Airport was backed by Prime Minister Theresa May on Tuesday October 25.
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