The only clear beneficiary of an extra, third runway at the west London airport would be British Airways, a report by the Hacan ClearSkies organisation said.

It said there was evidence that Heathrow's "rival" airports - Amsterdam, Frankfurt and Paris -, actually needed each other, instead of being in all-out competition.

It said many passengers at Heathrow were merely transfers and never left the airport, so never contributed to the UK or London economy.

Gatwick plans

"Building an additional runway at Heathrow simply to meet the business plans of one airline is a form of state aid that favours one airline against others," said the report.

Hacan chairman John Stewart added: "The government seems to be colluding with the airlines to up their profits whatever the cost to London's economy and the health of the people under the flight path."

A BAA Heathrow spokesman said: "Transfer passengers are important to Heathrow and the regional economy.

"Also people from all over this country are able to reach the foreign destination of their choice without transferring abroad.

"Heathrow has one of the lowest proportions of transfer passengers among European airport hubs because capacity constraints do not allow for more."

A third runway at Heathrow is one of the options being considered by the government following the publication of its proposals last summer for nationwide aviation expansion.

There were no plans for Gatwick in the original proposals, but after losing a legal battle, the government is now bringing out further consultation documents which will include possible options for the West Sussex airport.