Tony Tyler, himself a Briton, told a briefing at IATA's headquarters that the extra runway is vital even if a proposed long-term project to build a sixth London airport goes ahead.

Heathrow is running at nearly full capacity and Tyler, who headed Hong Kong's Cathay Pacific before joining IATA in 2011, said: "Even if you decide (a new airport) is necessary, it will take such a long time to build that you have to do something - and it seems to me that adding a new runway at Heathrow would be a sensible thing to do."

The statement placed IATA and its 240 airline members in the middle of a battle between London Mayor Boris Johnson and Prime Minister David Cameron.

The Conservative-led coalition government blocked a third runway when it came to power in 2010, but speculation is growing that Cameron would reverse that position if he wins the next election.

Johnson, seen by some as a potential future candidate to lead the Conservative party, wants to build a new airport east of the capital dubbed "Boris Island". Like many Londoners, he opposes a third Heathrow runway and the extra traffic that would fly over the British capital as a result.

Businesses have called for new capacity near London to increase the number of flights to large emerging markets and connecting flights from around Europe.

A recent study by Heathrow's owner Ferrovial showed that a lack of capacity cost Britain $22 billion a year in lost trade.

Aviation supported 1.4 million jobs in Britain and 3.6 percent of national GDP, Tyler said.

Apart from Heathrow, London's airports are Gatwick to the south of the capital, Stansted and City to the east and Luton to the north. (Editing by David Goodman)

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