Boris Johnson's dreams of moving Heathrow airport to the Thames Estuary took a major setback today after an investigation found it would cause huge environmental, financial and safety risks.
A study for by the Airports Commission found that a new estuary airport on the Hoo Peninsula in Kent, would cause "large scale direct habitat loss" to hundreds of thousands of migrating birds.
The cost of creating replacement habitats could exceed £2 billion and may not even be possible, the report found.
Even if replacement habitat could be found, planes using the airport would still be at a "high risk" of lethal bird strike.
In order to counter this risk, even larger areas of habitat would need to be destroyed to secure the airport.
Johnson was today urged to abandon the project.
"The Airport’s Commission has confirmed what a costly environmental disaster the mayor’s Thames Estuary Airport represents," Green Party London Assembly member Darren Johnson said.
"The Mayor needs to abandon this ill-conceived project."
'If politicians continue to dither on a decision on airport capacity we will start to prejudice London's premier position'
The report also found huge regulatory hurdles to any potential estuary airport going ahead.
Under environmental regulations, backers of the airport would first need to prove that there were "imperative reasons of overriding public interest (IROPI)" for placing the airport in such an environmentally sensitive area.
Even if that could be proven, they would also need to demonstrate that all of the habitat displaced by the airport could be placed elsewhere.
The report found that while this was "technically possible," it was highly uncertain, as such a large scale displacement had never been attempted before.
"It is technically possible but the scale of the required compensation is unprecedented to date and there is a high level of uncertainty given that the full requirement is yet to be understood," they found.
They also found that any attempt to create new habitats nearby would leave the airport vulnerable to bird strike, leaving great uncertainty about any alternative sites being found.
The report found that the airport would also cause new flood risks and leave the infrastructure vulnerable to the effects of climate change and rising sea levels.
The findings are the latest blow to Johnson's hopes of persuading the government of his case for a new hub airport in the Thames Estuary.
The mayor has spent more than six years and tens of millions of pounds in pushing the case for the airport.
Despite this, the Airports Commission chairman Howard Davies failed to include the plans on his shortlist of options for expanding airport capacity last year.
Davies did submit the plans for further investigation by the commission however, following heavy lobbying from City Hall.
The commission's latest report is unlikely to persuade David Cameron of the case for an estuary airport.
Experts have put the final cost of a new airport in the Estuary at around £50 billion, far more than alternative schemes.
The prime minister is believed to favour either expanding existing airports at Heathrow or Gatwick instead, but has put off any decision until after the general election next year.