More than a quarter of passengers at Britain’s most congested airport are overseas travellers who are stopping only briefly to change planes, according to figures obtained by The Times.

The number of passengers arriving at Heathrow from abroad and transferring to another international flight has trebled since 1991 to 18 million a year, and is expected to be double that by 2030. This costs the Treasury millions in lost potential revenue because transfer passengers do not pay air passenger duty.

Such travellers spend only a few pounds each in the departure lounges but are highly profitable to British Airways, which operates 40 per cent of Heathrow’s flights, because they help to fill empty seats. But every seat sold to a foreign transfer passenger costs the Exchequer up to £80 in lost revenue.

If all their seats were occupied by people either starting their journeys at Heathrow or transferring from a domestic flight, the Exchequer would gain more than £500 million a year.

The figures call into question the Government’s plan, due to be confirmed this summer, to permit the demolition of 700 homes to make way for a third runway at Heathrow. The capacity of the runway, which would open in 2020, would be equal roughly to the number of foreign transfer passengers in 2030.

Airlines argue that foreign transfer passengers are essential to maintain the high number of destinations served by Heathrow. But the rapid growth in their numbers has coincided with a decline in destinations, down 21 per cent since 1990 from 227 to 180.

Ruth Kelly, the Transport Secretary, says that Heathrow must expand because it is Britain’s “main gateway to the global economy”. She argues that better international connections are needed to attract companies to invest in Britain and to protect London’s status as a leading financial centre.

But figures from the Civil Aviation Authority reveal that, in the past 15 years, the proportion of seats on Heathrow flights taken by foreign transfer passengers has grown at the expense of British passengers and foreign visitors.

In 1991 16 per cent of Heathrow’s passengers switched from one international flight to another. By 2006 the proportion had risen to 26 per cent and is forecast to rise to 31 per cent by 2010. Norman Baker, the Liberal Democrat transport spokesman, said: “These are devastating figures which show that the third runway, for all its massive cost and environmental damage, is being built to help international transfer passengers who never leave the airport. If the number were reduced, we wouldn’t need a new runway and we would have more space at Heathrow for people from this country. With ever-increasing demand from British passengers to use Heathrow, we don’t need foreign transfer passengers to make routes viable. They are there simply to satisfy the greed of BA and BAA [the Spanish-owned airport operator].”

A BA spokesman said: “Without seat purchases from overseas transfer passengers, many long-haul routes from Heathrow would be unviable.

“That would mean that passengers in London and the South East would lose direct flights to a swath of destinations, putting businesses here at a serious disadvantage.”

Lord Soley, director of Future Heathrow, a lobby group supporting the third runway and funded by the aviation industry, said the airport needed to attract foreign transfer passengers to compete with rival international hubs in Paris, Frankfurt and Amsterdam.

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