Foreign Secretary Boris Johnson has said a third runway at Heathrow is "undeliverable", adding: "I think it very likely it will be stopped."
The long-time opponent of Heathrow expansion has been given permission to publicly speak out against the Government decision.
Mr Johnson said the project would cause "inevitable degradation" to the quality of life of people under the potential new flight paths.
"A third runway is undeliverable," he said. "The day when the bulldozers appear is a long way off, if indeed they ever materialise."
Tory MPs are taking lumps out of each other on Heathrow, grammar schools and - above all - the true meaning of Brexit.
Environmentalist Goldsmith called the move "catastrophic" - and announced he will now stand as an independent in protest.
The expansion of the London airport will affect the whole of the UK, with more domestic routes, while impacting air fares and house prices.