More than a third of Heathrow users are transfer passengers
Britain's busiest airport, Heathrow, is "jam-packed" and needs a third runway to remain competitive globally, the head of its operator, BAA, has said.
In an aviation conference speech, Colin Matthews also rejected a call to cut the number of transfer passengers to lessen the load on the airport.
A report from business group London First has called for improvements before Heathrow becomes bigger.
But BAA's new chief executive said that this was "a false choice".
"We do need the new capacity at Heathrow today in order to maintain its role, the way London and the country connects to the rest of the world," Mr Matthews told BBC News ahead of his speech to industry professionals in London. "If we don't have transfer traffic, we can't fulfil that role."
A government decision on whether to go ahead with a third runway is expected later this year.
Environmentalists have always been strongly opposed to the idea, and the Conservative leader David Cameron recently questioned the need for a new runway.
Campaigners opposed to the planned expansion say it would have a serious impact on hundreds of thousands of homes in the area.
More than a third of Heathrow's passengers are transfer passengers. Of the major European airports, only Frankfurt carries more transit passengers than Heathrow.
And that number has been increasing. Last year, 23 million people landed at Heathrow before taking off for another destination, up from 19 million in 2000.
Reducing transfer passenger numbers would be a "fundamental error", Mr Matthews said.
"Does anybody seriously think that if people living and working not just in London, but in the rest of the country, were forced to go to Charles de Gaulle airport or Schiphol airport to fly to the rest of the world, our economy will not suffer?" he argued.
In contrast, the business lobby group London First said the priority should be improving the existing airport before a decision to expand was taken.
In a report published on Wednesday, it suggested this could involve reducing the number of flights or using operational changes at Heathrow to release the pressure on the airport.
"Heathrow has been turned from a silk purse to a sow's ear," Baroness Jo Valentine, London First's chief executive, said.
"Once an asset in attracting business to the capital, it is at risk of becoming a liability."
BAA is also facing a ruling from the Competition Commission relating to BAA's ownership of London's three biggest airports.
Heathrow has not kept pace with competitors like Schiphol, BAA says
There is speculation that it will be forced to sell off one of its airports to improve competition between airport operators and improve choice for travellers in the south-east of England.
However, Mr Matthews told the BBC that the inquiry was "focusing on the wrong issue".
"The right issue is the provision of the right capacity at the right time," he said.
"Changing the ownership structure could delay this country facing up to those difficult questions of putting in the right capacity at the right time," Mr Matthews said.