YouGov polled more than 1,000 adults last month, finding that 46 per cent preferred expanding Gatwick Airport. Thirty-four per cent wanted Heathrow to be expanded, instead.
YouGov also found that Gatwick outpolled Heathrow on multiple criteria, including which airport's expansion would lead to less pollution, encourage more competition and be built at a lower cost to taxpayers.
“Time after time Londoners say Gatwick expansion is best for the environment, for the taxpayer, regeneration, competition and for speed of delivery," said Gatwick chief executive Stewart Wingate. "These very same Londoners would be forced to live with illegal air quality levels if Heathrow expands, so their views need to be taken seriously.
“‘Momentum is with Gatwick as more people recognise it is the only solution that balances the economy and the environment.
"It is also the only plan that is actually legal, so if we want Britain to get on with it the answer is obvious," Wingate added.
The Airports Commission, led by Royal Bank of Scotland (RBS) chairman Sir Howard Davies, first recommended building a third runway at Heathrow last July. The government said in December that it would delay making a decision on airport expansion in the south east until at least this summer, saying "more work will be done on environmental impacts" in the interim.
Transport secretary Patrick McLoughlin has repeatedly said that the government is open to a second runway at Gatwick.
A Heathrow spokesperson rejected today's figures, saying: "Airport expansion is an issue for the whole of the UK. That's why Heathrow has backing across Britain from business groups, trade unions, exporters, a majority of MPs across the political divide as well as two-thirds of local residents, as the only option which delivers up to £211bn in economic benefit and 180,000 jobs up and down the country.
"A third runway will be the UK's best value for money infrastructure project, giving Britain the most connected and sustainable hub airport in the world by adding more than 40 direct long-haul routes whilst meeting environmental and legal air quality limits," the spokesperson added.