A review in 2015 recommended adding a new runway at Heathrow as the best option; advocates of expansion argue that, with Britain planning to leave the European Union next year, it has never been more important to improve air capacity to destinations outside Europe.
So, after years of equivocation, this month Mrs. May finally threw her personal support behind the Heathrow expansion plan.
Before the vote on Monday, John Holland-Kaye, the chief executive of Heathrow, said that an expansion “will connect all of Britain to global trade, increase competition and choice for passengers and create tens of thousands of new skilled jobs for future generations.” Len McCluskey, general secretary of the influential Unite trade union, echoed that message.
But while some of those living near the airport depend on it for their livelihoods, many others have campaigned against a project that threatens to destroy some settlements like Harmondsworth, a village that dates from medieval times.
“Local communities are clear that the costs of expansion at Heathrow, including as many as 700 more planes in the skies every single day, far outweigh any alleged benefits,” said Paul McGuinness, who leads the No 3rd Runway Coalition, a group opposed to expansion. Mr. McGuinness added that the project was “the costliest option considered by government, carries the largest financial and planning risk, and has the most significant and negative environmental impact.”
Many Londoners are plagued by aircraft noise, and critics of the Heathrow plan include Mayor Sadiq Khan, who supported a rival proposal to expand Gatwick.
“This will be a critical moment,” Mr. Khan said before Monday’s vote. “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.”