25 October 2016 Last updated at 14:21 BST
Mr Johnson, who campaigned against the proposal while running for mayor of London, said there could eventually be a "clamour" for a fourth runway.
But he predicted that there was little chance of the third runway being built in the first place.
The government has given Mr Johnson permission to campaign against its stance on Heathrow.
UK Foreign Secretary and long-time opponent of Heathrow expansion Boris Johnson says that while New York is known as the "city of skyscrapers" and Paris the "city of light", London risks becoming the "city of planes" if the third runway goes ahead.