Friends of the Earth said the Thames Estuary airport scheme was a "pie-in-the-sky plan" and that Mr Johnson should "stop pitching communities in west London against those in Essex and Kent and back transport solutions that don't wreck our planet".
Rodney Chambers, leader of Medway Council in Kent, said: "For five years now, Boris has failed to listen to the huge number of people who have said that placing an airport in North Kent, on or near the Thames Estuary, is a complete non-starter."
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.