Airline chiefs today condemned Boris Johnson's plans to close Heathrow and open a new airport on an artificial island on the Thames estuary.
British Airways and Heathrow's owner BAA said closing Heathrow would cost 77,000 jobs and threaten London's position as an international transport hub.
Environmental groups also warned the plans could be "catastrophic" for wildlife in the sensitive and legally-protected estuary habitat.
Mr Johnson, who opposes building a third runway at Heathrow, has ordered a detailed study into building a four-runway international hub on a massive artificial island two miles off the Kent coast near Sheppey.
The proposed airport, similar to a £31.5 billion scheme first suggestedmore than five years ago, could operate 24 hours a day as planes would approach over the North Sea, away from residential areas, and would be built to cope with a predicted doubling of passenger numbers in the next two decades.
City Hall officials believe it could be built in six years at a cost of between £30 and 40billion, and are understood to be drawing up plans to ferry passengers to and from London in 35 minutes via a high-speed rail link.
A review has been completed and an engineering company is now conducting a feasibility study, the Mayor has confirmed.
His planning team, led by deputy mayor Kit Malthouse, hope it would lead to the eventual closure of Heathrow, which could be redeveloped as a hightech business park.
Mr Malthouse said: "I think it's madness to expand any of the other airports when there is an obvious solution elsewhere. If we can build St Paul's, the Gherkin, the Channel Tunnel and all the rest of it, then we can do this. We just need a bit of courage." But the proposals, already dubbed "Boris Island", have met instant opposition from politicians, campaign groups and airline bosses.
Baroness Jo Valentine, chief executive of business lobby group London First, called it "an idealistic solution which could take 30 or 40 years to develop".
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BAA said: "What London and the UK need now is new runway capacity if we are to avoid losing our connections to the world, particularly India and China."
A BA spokesman said: "It would cost a vast amount. Where would the funding come from?" Strikes planned for tomorrow and Thursday by workers who operate luggage scanning machines at Stansted were called off today to allow fresh talks in a bid to resolve the pay row.
