EXPANDING Heathrow airport may spark Britain’s biggest-ever single environmental campaign, former shadow chancellor John McDonnell warned today.

The MP for Hayes and Harlington, which contains Heathrow, was commenting as a new expert report rubbished claims that a third runway would deliver economic growth amid reports that the Chancellor is preparing to back the project.

Mr McDonnell said: “Backing a third runway at Heathrow would shred the government’s credibility in one fell swoop and potentially launch the greatest single environmental campaign this country has seen.