Dunlin are found in huge numbers in the Thames Estuary
Increasing numbers of wildlife and birds in the Thames Estuary could end plans for an airport on an artificial island, the RSPB has claimed.
Its Waterbirds in the UK 2006/2007 report places the estuary in the top five most important sites in the UK for water birds wintering or migrating.
The RSPB said it proved plans for an airport in the estuary were doomed.
London Mayor Boris Johnson proposed building an airport in the estuary in his mayoral manifesto.
The RSPB began counting water bird numbers in the 1960s in response to the proposed Maplin Sands Airport in Essex.
It said this year's count has found 12 species in the Thames Estuary in internationally important numbers defined as 10% of the global population and this is the highest concentration anywhere in the South East of England.
Chris Corrigan, RSPB South East's regional director, said: "If ever Boris needed proof of the environmental cost involved in building a Thames Estuary airport, this report is it.
"For years we have been pointing to the estuary's importance for countless species and here, in black and white yet again, is proof of just how remarkable the area really is for wildlife.
"The nearby Swale and Medway Estuaries, similarly recognised by this report for their international importance, will also lose out if an airport went ahead."
The study found the Thames Estuary to be the only internationally important site in the UK for the amber-listed ringed plover, the RSPB said.
It is also the second most internationally important site in the UK for dunlin, which can be found in huge numbers in the area but are found to be at their lowest level nationally since the 1970s.
Globally significant populations of the UK's two species of godwit - types of wading bird - were also found in the Thames Estuary.
In his transport manifesto during the mayoral elections Mr Johnson spoke about a possible new airport in the Thames Estuary.
Mr Johnson said: "You can't endlessly expand Heathrow in the suburbs of west London and entrench what was really a planning error of decades ago.
"I'm looking at all the airports around the perimeter including the option of a new site somewhere in the Thames Estuary, that's something I definitely think we should look at."