"This is a really important moment in the history of this House and the history of this country," Transport Secretary Chris Grayling told the House of Commons as he appealed for lawmakers to "move on from decades of debate and set, to my mind, a clear path to our future as a global nation in the post-Brexit world."Opponents object to the third runway on environmental, noise and financial grounds. Friends of the Earth described it as a "morally reprehensible" move that would result in Heathrow emitting as much carbon as the whole of Portugal.

Greenpeace said that if ministers wouldn't protect people from toxic air, opponents would ask a court to do so.

John Stewart, a longtime opponent of Heathrow expansion, took to Twitter to lament that the new runway had become a government policy.

"A third runway will turn peaceful areas of London & the Home Counties into torrents of noise as planes pass over at a rate of 1 every 90 seconds," he said of the towns under the airport's flight path.

London Mayor Sadiq Khan has pledged to join local government councils in filing legal action seeking to block the expansion and has said Heathrow already exposes the city to more aircraft noise than Paris, Frankfurt, Amsterdam, Munich and Madrid combined. He argues that the project would also push toxic emissions above legal limits.

"This will be a critical moment, and for the sake of Londoners affected by poor air quality, disruption from noise and the costs needed to improve transport connections, I will do what I can to stop these poor plans," Khan said in a statement.

May directed Conservative Party lawmakers to vote for the project. Foreign Secretary Boris Johnson, who once pledged to lie down in front of bulldozers to stop the expansion, avoided a confrontation with the prime minister because he was visiting Afghanistan on Monday.