Johnson, who was previously mayor of London, has been a vocal opponent of plans to expand Heathrow for years. He sought and was granted permission to publicly oppose the government's decision on the issue.

"I do think that building a third runway slap bang in the middle of the western suburbs of the greatest city on Earth is not the right thing to do," Johnson said.

"What I worry about is that down the line, if and when a third runway were to be built, and I don't think it would be, but suppose it were to be, there would be an overwhelming clamour to build a fourth runway as soon as it was completed and then what would London be like?"

"You'd have New York, a city of beautiful skyscrapers, Paris, the city of light, London, the city of planes. Is that really what we want for our fantastic capital city?"

(Reporting by Estelle Shirbon and Costas Pitas)