Plans for a new airport in the Thames Estuary have been given their biggest push yet by London mayor Boris Johnson.
In a new document for senior leaders at the Greater London Authority (GLA), he says the airport could be the "single biggest... and bravest project we do".
But only last month Mr Johnson told the GLA's transport committee that he had no aspiration to build such an airport.
His spokesman said: "There is a huge amount of work before an airport could go through the planning process."
"The mayor has been clear that his mayoralty will take a creative and open-minded approach to the challenges facing the capital."
The document, circulated to senior colleagues in the past four weeks and seen by the BBC, sets out Mr Johnson's plans for his time as London mayor, including his airport strategy.
The mayor believes the complex and critical decisions on Britain's aviation future require mature exploration of every possible option
The mayor first proposed building an airport, which has become known as "Boris Island", in his mayoral manifesto.
Plans have progressed to the point where last month he released a feasibility study in which civil engineer Doug Oakervee found that there were no "overwhelming constraints".
But on 20 October he told the GLA transport committee, under questioning by deputy chair Valerie Shawcross: "It is very important that we clear up the status of this feasibility study and the status of this airport.
"I do not have an aspiration to construct such an airport."
City Hall officials have said previously that the estimated £40bn cost could be met entirely with private funding.
Deputy mayor Kit Malthouse said in October that international financiers had shown "considerable interest" in the project.
Kent County Council has said it will fight plans for the airport, near the Isle of Sheppey.
The RSPB is also against the project on environmental grounds, saying the estuary is one of the five most important sites in the UK for water birds wintering or migrating.
Mr Johnson's spokesman said the mayor believed the airport would be extremely valuable to London and he had set up a Thames Estuary Steering Group, including members from industry, business, and from across the political spectrum.
"They will oversee further study and research on issues such as energy, flood management, transport, ecology and regeneration," he said.
"The mayor believes the complex and critical decisions on Britain's aviation future require mature exploration of every possible option."
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