Boris Johnson and Doug Oakervee visited the proposed site in 2009
Plans by London's mayor to build an airport on an artificial island in the Thames estuary have been branded "pie in the sky" by Medway Council.
A feasibility review approved by the council's cabinet says Boris Johnson's plan is "completely wrong".
It says the airport would be some 60 miles from London and would be like moving Manchester Airport to Leeds.
Mr Johnson's spokesman has said the mayor believes the airport would be extremely valuable to London.
The mayor has set up a Thames Estuary Steering Group, including members from industry, business and from across the political spectrum to look into the airport, which would cost at least £40bn.
Medway's feasibility review says that, if built, the airport would be in an area where aircraft would be 12 times more at risk of bird strike than any other major UK airport.
It would harm Sites of Special Scientific Interest and other areas used by hundreds of thousands of migrating birds annually.
It says it would be too close to Thamesport, where ships unload cargoes of liquid natural gas.
And it raises concerns that a new airport would encourage air travel and create a need for major road and rail links which would harm Medway's landscape.
Medway Council is campaigning to stop the airport, with Kent County Council and the Royal Society for the Protection of Birds.
It started a joint online petition last year.
Mr Johnson believes the estuary airport is an alternative to government plans to build a third runway at Heathrow.
Last year he released a feasibility study by Doug Oakervee, who helped construct a similar airport in Hong Kong, which came out in favour of the idea.
The steering group will oversee further study and research on issues such as energy, flood management, transport, ecology and regeneration.