The government is to hold a formal consultation on controversial plans for a new airport in the Thames Estuary.
The consultation on the airport, dubbed “Boris Island” because of its backing from London Mayor Boris Johnson, is expected to be announced in March.
Downing Street said no decisions had been made but ministers wanted to explore all options for maintaining the UK’s status as a global aviation hub.
The prime minister has previously ruled out any further expansion to Heathrow reports the BBC.
In his Autumn Statement, Chancellor George Osborne did not rule out a new hub airport in the Thames Estuary, and this was seen as a clear sign that the government was warming to the scheme.
Prime Minister David Cameron is said to be supportive of Mr Johnson’s alternative to expanding Heathrow, but will await the outcome of the consultation.
Sources close to Mr Johnson said the idea of an airport to the east of London, which would be built partly on reclaimed land, had been gaining ground in conversations with ministers and a formal consultation was a major step forward.
Passenger demand for London’s airports is forecast to increase from 140 million passengers a year in 2010 to 400 million passengers a year by 2050, according to a previous report by the Greater London Authority.
Mr Johnson has said the constrictions of Heathrow were becoming “ever more apparent and ever more damaging”.
However, concerns have been raised about damage to the environment.
The RSPB, Medway Council and Kent County Council have opposed the Thames Estuary idea, saying it is “undeliverable, unaffordable and unnecessary”.
In May last year plans for a third runway and sixth terminal at Heathrow were scrapped when the coalition government took office.