Four councils whose residents will be affected are planning a joint legal challenge to the decision. They will argue that the expansion will lead to a breach of EU rules on nitrogen oxide pollution. They include Windsor and Maidenhead, where May is the local MP. She has in the past opposed a third runway, on the grounds of its impact on her constituents. To build the runway, 750 homes will have to be compulsorily purchased, and the owners of a further 3,750 will be given the option of selling their properties to the airport. They will all be offered their homes at “unblighted” market value, plus 25% – at an estimated cost of up to around £1.5bn.