Five local government councils are going to court to stop the expansion of London's Heathrow Airport.
London Mayor Sadiq Khan has joined the action, which began today when five boroughs notified the national government of the legal grounds for their action.
Parliament last month approved plans for a third runway at Heathrow, backing what the government described as the most important transportation project in a generation.
Prime Minister Theresa May says the expansion will boost economic growth, but neighbours and environmentalists object because of concerns about noise, pollution and climate change. Khan says the government failed to "demonstrate how it will fund the billions of pounds needed to improve road and rail connections to the airport".