Boris Johnson has predicted "clean, green aviation" is likely to happen before a third runway is built at Heathrow Airport.
The Prime Minister's remarks cast further doubt on the expansion of the west London hub after he was challenged in the House of Commons.
Labour MP Rupa Huq (Ealing Central and Acton) had asked for a fresh vote to allow MPs "to kill off" the project.
Mr Johnson said the UK government was focused on achieving "net-zero aviation", adding at Prime Minister's Questions: "That's the future of this country: clean, green aviation.
"And by the way, I think that has every chance of arriving a lot earlier than a third runway at Heathrow."
Mr Johnson – who is MP for Uxbridge and South Ruislip – has previously voiced his opposition to expansion at Heathrow, declaring that he would lie down in front of the bulldozers.
Geraldine Nicholson, from the Stop Heathrow Expansion campaign group, said in a statement: "The prime minister knows that Heathrow expansion would so badly undermine our ability to meet new climate legislation, but also would result in toxic levels of air pollution in areas around the airport.
"Together with the government's pledge not to commit a penny in funding for the project, this is surely the end of it.
"It is the news our communities and the wider environment deserve."
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