A mystery consortium is drawing up plans for a new, four-runway airport close to Heathrow, it has been reported.
The unnamed group of British businesses has commissioned a "world-leading infrastructure firm" to assess potential sites to the west and north of London which could rival - or even replace - Heathrow, according to The Independent on Sunday (IoS).
The paper said that the consortium was understood to have opened talks with Chinese sovereign wealth funds over the financing of the project which could cost between £40bn to £60bn.
The report comes as Transport Secretary Justine Greening is preparing to launch a major consultation on future airport expansion amid warnings from business that the country cannot carry on with the present level of provision and remain competitive.
According to the IoS, the feasibility study is being carried out by a firm of specialist engineers "who have worked on major aviation projects in North and Latin America".
They were said to have been tasked to evaluate flat tracts of land which could take such a large development, with sites in Berkshire and Oxfordshire potentially in the frame.
Such a proposal would be highly controversial and would inevitably face fierce opposition from communities and environmental groups.
The IoS said that documents it had seen emphasised the importance of building a political consensus behind any proposal.
"As evidenced by HS2 (High Speed Rail), Crossrail, and the London 2012 Olympics, the development and delivery of any scheme must have cross-party backing and must be supported by business and the workforce."
The paper said that the group behind the scheme was expected to reveal itself within weeks.
It quoted an industry source as saying: "These are very serious people. They want all their ideas aligned before coming forward publicly - this is going to be pretty impressive stuff."
Foreign Secretary William Hague firmly ruled out a change of Government on policy on a third runway at Heathrow.
Speaking to the Sky News Murnaghan programme, he said: "The circumstances have not changed... it's important to stick to that election promise.
"It's important to make the right decision about this and study all the options.
"We said very specifically we would not be (building a third runway). Because a runway or an airport takes such a long time, this does not have an immediate effect on the economic situation now and our airport capacity now, but there are others things which do have an effect."