It would include the UK's busiest railway station, handling 300,000 passengers a day.
Planes would fly in from the north-east, primarily over water, greatly reducing the number of homes that would be affected by aircraft noise.
With the Government having admitted in August that London's main airports will reach bursting point in 2030 - and a third runway at Heathrow having been ruled out - backers of Thames Hub insist it offers a splendid solution.
But they have yet to convince everyone, especially people in the area.
Councillor Rodney Chambers, leader of Medway Council in Kent, said: "The Isle of Grain is home to one of the world's largest liquefied natural gas terminals, with a fifth of the UK's gas supply offloaded by container ships and stored there.
"We have looked at Lord Foster's plan and he appears to want to place his fantasy Isle of Grain airport on top of the LNG plant and a power station. It beggars belief."
Lord Foster said: "We need to recapture the foresight and political courage of our 19th century forebears if we are to establish a modern transport and energy infrastructure in Britain for this century and beyond. We can do it here."
The airport's backers claim that if the Government supports the scheme, sovereign wealth funds and wealthy foreign investors will stump up the cash to make it a reality.
The Mayor of London, Boris Johnson, has declared his support for the plans, despite them rivalling his own vision for a "floating" airport in the Thames Estuary.
A spokesman said: "He is delighted that a distinguished figure like Lord Foster agrees that the answer to Britain's aviation needs lie in the (Thames) estuary."
Proposed site of the world's largest airport
View Proposed site of world's largest airport in a larger map
- DAILY MAIL
