Easyjet has thrown its support behind London Heathrow's plan for a third runway.
The budget carrier made its position known in a submission to the Airports Commission, which is looking into how to expand the UK's airport capacity.
It comes as a blow to Gatwick, which has been campaigning for a second runway and is Easyjet's biggest base.
In Easyjet's submission, CEO Carolyn McCall said: "Heathrow is in the best interests of passengers as it has the greatest demand. It is clear that long-haul airlines want to expand at Heathrow and if they can't, they will do so not at Gatwick but at other airports such as Paris, Amsterdam and Frankfurt."
Easyjet is in favour of expansion at Heathrow as it will allow short-haul airlines to operate out of Europe's busiest airport.
Gatwick has responded by accusing Easyjet of taking a "position based on its own narrow commercial interests".
A statement said: "The government, however, will have to make a decision for the country balancing what is best for both the economy and the environment. That can only mean Gatwick.
"Gatwick can deliver its second runway without the massive environmental damage which has stopped Heathrow expansion time and time again. That means Britain can finally get on with it. The economy can grow without a new population the size of Coventry severely affected by noise. And fares can stay low for everyone."
Last week, West Sussex County Council became the second local authority to withdraw its support for a second runway at Gatwick (see news, January 20).
It was also announced last week that Easyjet will move its entire operation at Gatwick into the North Terminal next year (see news, January 23).