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 not 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."
Although expansion at Heathrow will remain the bookies' favourite, Sir Howard said that deciding that London could best be served by "putting all your eggs in one basket and having one huge hub airport strikes us as quite risky".
He said that one thing that needed to be weighed carefully was the possibility of low-cost airline flights increasing and these budget carriers using long-haul aircraft.
Sir Howard said that Gatwick and Heathrow managements "don't agree on much" but one thing they were agreed on was that it would not be possible to build extra runways at the two airports at the same time.
He went on: "It's pretty clear to us that you need to make a choice of which of them goes first."
Sir Howard admitted that for residents at Gatwick and Heathrow there could well be "issues of blight" and that it was up to the Government and airport managements to consider this.
He said that of the two Heathrow options, the full-length new runway to the north-west was the "most attractive" while the Heathrow Hub scheme was "worthy of serious consideration".
Local groups say the north-west plan will require significant demolition in the villages of Longford and Harmondsworth.
Anti-Heathrow expansion group Hacan vowed today to fight the Heathrow plans. "We understand the strength of feeling of those living near Heathrow," Sir Howard said.
Shadow transport secretary Mary Creagh said Labour would now study Sir Howard's recommendations.
She went on: "It is vital that we take decisions about our airport capacity, including in the South East, which are important for Britain's competitiveness.
"As the commission now looks in greater detail at specific proposals, it remains crucial that they take into account the need to minimise local and environmental impacts of increased capacity."
Mr Johnson said expanding Heathrow would be "completely crackers" and create an "insatiable" demand for a fourth runway
He went on: "Why on earth entrench a huge planning error and expand Heathrow and consign future generations to misery when we could go for the right option?"
While the Stop Stansted Expansion group was thrilled the Essex airport was left off the immediate short list, the Gatwick Area Conservation Campaign greeted news of Gatwick's appearance on the short list with dismay.
Medway Council in Kent was also unhappy that the Thames Estuary plan had not been ruled out.
West Sussex County Council said it was pleased that Gatwick was on the short list but the Green Party and environmental groups all expressed alarm at the commission's findings.
Sir Howard's team's conclusions were welcomed by airlines and big business.
Mr Johnson insisted that Heathrow would be "the wrong way forward for the country" and pledged to fight on to win support for his Thames estuary airport scheme.
The London Mayor called for "clarity" from the Government about whether it backs a third runway at Heathrow, which he said would not be politically "deliverable". And he insisted that, far from being holed beneath the water by the Davies report, the case for Boris Island was still "winnable".
He challenged Sir Howard's costings for the estuary airport, which the mayor said would total around £50 billion - £20 billion for transport links and £30 billion for the facility itself, which he predicted could be funded from private investment.
Mr Johnson told a Westminster lunch: "It's pretty obvious to me what is going on. I think the reality is that Sir Howard probably began with a short list that didn't really include much except Heathrow, and I think he has been told to have another think and that's good.
"I will work with that. I will seize what lifeline I can. I will keep the estuary option going. I will try to win this argument. I think it is winnable. It is vital that we do it.
"But we need to get on with it. If the Government, if the great mass of the British establishment is basically still addicted to the idea of a third runway at Heathrow, let's be clear about that, let's have it out there, let's have a proper debate about it.
"I think it would be an environmental catastrophe. I don't think it's deliverable. I think it's the wrong way forward for the country.
"But the sooner we have clarity the better and at the moment, I think we are being deprived of that essential clarity. No one can really discuss this in a very productive way because we don't really where the Government is."
Asked who he believed had spoken to Sir Howard to influence him to keep the estuary airport on the table, Mr Johnson replied: "Me. I told him.
"I don't think that he has been nobbled, because he has come up with a better idea, and there is much to be applauded in that.
"We need to develop that idea and we have six months now to make the case for the estuary and we are going to do it.
"The sooner we get a clear answer from the Government, in my view, the better. We can't keep pussyfooting and fannying around forever."
He added: "Everybody in my party and indeed in several other parties, so far as I can remember, were elected on a manifesto to oppose a third runway at Heathrow. That happens to be the correct policy. Why change it? Why dump it? The sooner we get back to that, the better."