Politicians are likely to have to decide between a new runway at either Heathrow or Gatwick following an interim report today by the Whitehall-commissioned Airports Commission.
Expansion at Stansted airport in Essex has been ruled out until after 2030 by the commission, which is headed by former Financial Services Authority chief Sir Howard Davies.
Sir Howard's team have not closed the door on a Thames Estuary scheme, which is favoured by London mayor Boris Johnson.
But although the commission will study estuary schemes further, Sir Howard described it as "imaginative", adding that it could cost as much as £112 billion, would need much more public money than the other options and the construction challenge would be "massive".
The commission concluded that there was "a need for one net additional runway to be in operation in the South East by 2030 and there was " likely to be a demand case for a second additional runway to be operational by 2050".
Sir Howard's team said it would be taking proposals for new runways at two locations forward for further detailed study:
:: GATWICK - This is the plan by the airport's bosses for a new runway to the south of the existing runway;
:: HEATHROW - Here there are two options - The first is Heathrow Airport Ltd's proposal for one new 3,500-metre (11,500ft) runway to the north west to add to the two existing runways. This would be a less-noisy option than the runway proposed in Labour's aviation White Paper and would avoid the loss of Sipson village.
The second Heathrow option is the one put forward by Heathrow Hub, a group of civil engineers which also includes former Concorde pilot Jock Lowe. Theirs is a proposal to extend the existing northern runway to at least 6,000 metres (20,000ft), enabling the extended runway to operate as two independent runways.
The commission said it had added the Thames Estuary options to the short list "as there were too many uncertainties and challenges" surrounding them at this stage.
Sir Howard's team will now undertake further study of the Isle of Grain estuary option in the first half of 2014 and "will reach a view later next year on whether that option offers a credible proposal for consideration alongside the other shortlisted options".
The commission also said there was likely to be a case for considering Stansted and Birmingham as potential options for any second new runway by 2050.
There are no firm long-term proposals in the commission's interim report. Those will come when the commission makes its final report in the summer of 2015 - after the next general election.
The last Labour government supported a third runway at Heathrow but expansion at the west London airport was ruled out by the coalition Government when it took power in May 2010.
Last week, Tory MP Zac Goldsmith said any decision by the Prime Minister to back Heathrow expansion would represent an "off-the-scale betrayal" and David Cameron would "never be forgiven in west London" . Today Mr Johnson said a third runway at Heathrow would be "completely crackers".
Transport Secretary Patrick McLoughlin has said the coalition's pledge was not to build a third runway "in this parliament" and stressed that any decision would come after summer 2015.
Launching the report in London, Sir Howard said the shortlist had been finalised last week and his team had "gone back and forward" over whether to keep the estuary schemes "in place".
Sir Howard was repeatedly asked whether he had to take on board any views from ministers.
He replied: "The short list has not been influenced by politicians."
Pressed on any discussions he might have had with politicians, Sir Howard replied: "I am not going to tell you what politicians tell me at private meetings."