The government agreed to assess Johnson's plan to use the Isle of Grain, 35 miles downriver from the capital, The New York Times reported. Johnson argues the airport, which would cost $80 billion, is needed because Heathrow and other airports around the city are at near capacity.
British newspapers have nicknamed the plan "Boris Island."
While Prime Minister David Cameron backs the airport, Deputy Prime Minister Nick Clegg, the leader of the Liberal Democrats, does not, the Daily Mirror said. Clegg is said to think Cameron, a Conservative, hopes the project will boost Conservative Johnson's re-election prospects.
"We don't believe there is an economic or environmental case for it," a Clegg spokesman said.
Environmentalists say the Isle of Grain is a major nesting site for avocets and marsh harriers -- and equally important as a feeding ground for birds that winter in the area, the Times reported.
"I think that where we are is that the government is increasingly interested in this idea," Johnson said. "I genuinely believe that they see not just the overwhelming aviation argument and the argument from international competitiveness in making sure that Britain has a hub airport that is viable for the long-term future."