London's mayor is to press ahead with the next stage of plans for an airport in the Thames estuary.
A report by an engineer is understood to have found no major obstacles to constructing the airport several miles off the Essex and Kent coasts.
Boris Johnson is expected to announce he is to proceed with more engineering and environmental assessments.
It has angered environmentalists and Kent County Council has said it would fight the plans.
Boris Johnson has vehemently opposed a third runway at Heathrow.
But he believes the demand for international travel is unstoppable and Boris Island airport - as it has become known - is the only viable long-term solution.
A feasibility study by former chairman of Crossrail, Doug Oakervee, who helped construct a similar airport in Hong Kong, is favourable to the idea.
City Hall officials also believe Mr Oakervee debunks some of the myths about the potential harm to birdlife and the environment.
The RSPB's 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.
It would be totally horrendous
Kent County Council leader Paul Carter
The bird charity said it proved plans for an airport in the estuary were doomed.
And Kent County Council leader Paul Carter said his plan was "environmentally and ecologically" a very bad idea and said he would fight any proposals to build an airport off the Kent coast.
"Environmentally and ecologically it's a very bad idea to build an airport off one of the most beautiful coastlines in the South East," he added.
"To say it will be an eyesore is the understatement of the year. It would be totally horrendous."
Operator BAA says Heathrow Airport is "jam-packed"
Some estimate such an airport would cost at least £40bn, but City Hall officials have said previously that it could be built entirely with private funding.
Deputy Mayor Kit Malthouse said last week that international financiers had shown "considerable interest" in the project.
There has also been speculation that Chinese investors and Arab sheiks are waiting in the wings.
The most expensive single component of the project would not be the airport complex itself but the construction of a high-speed rail-link connecting it to land and taking passengers through the area known as the Thames Gateway.
Conservative MPs in Kent and Kent County Council are among those opposed to the idea.
David Cameron and Conservative Transport spokeswoman Theresa Villiers have not backed the plans, but say they have been content for the mayor to conduct a feasibility study.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.