British Conservative backbencher Zac Goldsmith has resigned as an MP in protest at the Heathrow runway decision, which he called "catastrophic".
The former candidate for London mayor stepped down with immediate effect, triggering a by-election in his Richmond Park and North Kingston constituency.
After the controversial decision was announced, he told MPs in the Commons the decision was "catastrophic".
"The government has chosen a course that is not only wrong, it's doomed," he said.
Mr Goldsmith is a well-known environmentalist whose wealthy southwest London constituency is under the Heathrow flight path.
Meanwhile, Foreign Secretary Boris Johnson has accused Prime Minister Theresa May of promising the "undeliverable" as the political fallout continues.
Mr Johnson said expansion at the West London airport would "very likely" be stopped and "the day when the bulldozers appear is a long way off, if indeed they ever materialise".
Campaigners, including Mr Johnson and London mayor Sadiq Khan, also warned the legal obstacles to getting the scheme off the ground would stop it going ahead by 2025 - if it ever did.
The government attempted to dampen the blow of it approving the £17.6bn project by attaching conditions including a six-and-a-half hour night-flight ban, more stringent noise conditions and a £2.6bn compensation fund for residents.
Speaking to the Evening Standard after the decision, Mrs May said she wanted the runway delivered "as quickly as possible".
She said: "After decades of delay we are showing that we will take the big decisions when they're the right decisions for Britain, and we will ensure they're right for ordinary working people too.
"Airport expansion is vital for the economic future of the whole of the UK and today also provides certainty to Londoners.
"Businesses will know that we are building the infrastructure they need to access global markets. Ordinary working people will know that my Government backs jobs and growth."
However, Boris Johnson, who along with other members of the Cabinet has been given permission by the PM to speak out against the decision for a limited time, said: "No other great city would do this to its inhabitants.
"New York is going to be the city of beautiful skyscrapers, Paris the city of lights and London in the future, if we go ahead with this project, will be known as the city of planes."
Mr Johnson, who has previously threatened to lie in front of bulldozers rather than let the scheme go ahead, added: "A third runway is undeliverable.
"The day when the bulldozers appear is a long way off, if indeed they ever materialise."
His comments came as the chief executive of Heathrow, John Holland-Kaye, said work on the runway would start in 2021 and would be open for business in 2025.
Business leaders, airlines and unions welcomed the news. Councils in the areas affected by the expansion have joined forces and announced their intention to launch a judicial review of the decision.
