Sky News understands that big investors in FGP Topco, the airport’s parent company, are refusing to back Chris Grayling’s suggestion that Heathrow gives a written commitment to implementing a proposal from Heathrow Hub, a rival.
Mr Grayling made the request at a meeting with the two runway promoters last month, since when further talks have been held between executives at Heathrow and Heathrow Hub.
Sources said that John Holland-Kaye, Heathrow’s chief executive, was “constructive” about the idea of backing the cheaper Heathrow Hub plan if ministers recommend it in the coming weeks.
However, insiders said on Tuesday that Mr Holland-Kaye had been told by his shareholders that acknowledging any support for the rival scheme would be a tactical error so close to an announcement by the Government.
Expansion at Gatwick Airport remains another option under consideration by ministers as they seek to resolve the long-running debate about Britain’s aviation capacity crunch.
FGP Topco’s shareholders are led by Ferrovial, the Spanish infrastructure group, which has a 25% stake.
Sovereign wealth and pension funds from Australia, Canada, China, Qatar and Singapore own the bulk of Heathrow’s shares.
Heathrow argues that it has not done sufficient due diligence to justify giving its backing to Heathrow Hub, whose controlling investor has pledged to hand to charity any profit he makes from licensing the idea to the airport’s owner.
Sources said, though, that the objection of some of Heathrow’s shareholders to Mr Grayling’s request stemmed from the fact that future financial returns would be lower if a cheaper expansion scheme was given the go-ahead.
The charges that airports are allowed to impose on customers are dictated by the Civil Aviation Authority’s calculation of their Regulated Asset Base, a figure which would be significantly higher if Heathrow builds a new third runway.
A Heathrow Hub spokesman said: “We have always enjoyed cordial relations with Heathrow and its management.
“We think it is inconceivable that if the Government recommended our cheaper and simpler proposal, that Heathrow’s shareholders would refuse to implement it.”
Sky News revealed during the summer that Anthony Clake, who oversees roughly $10bn at Marshall Wace Asset Management, is the majority shareholder in Heathrow Hub.
Mr Clake has pledged to cap the cost of licensing the scheme at £5m-a-year, removing a potential objection from the owner of the site.
The Government is expected to announce its preferred location for a new runway in mid-October, paving the way for a giant infrastructure project which the private sector has argued is essential to demonstrating that Britain is “open for business”.
Heathrow Hub had been viewed as the rank outsider following Sir Howard Davies’ recommendation last year that the airport should be able to build a third runway.
Gatwick, which has prominent backers in the form of the new London Mayor, Sadiq Khan, is also lobbying furiously on the grounds that a new runway there would cost less, cause less environmental and noise pollution, and deliver many of the same economic benefits as its larger rival.
In recent weeks, however, support has begun to build for the Heathrow Hub scheme, with the parent company of British Airways recently arguing that it should be given serious consideration.
Lord Maude, the former trade minister, has also given his support to Heathrow Hub.
Mr Clake previously said: “The Heathrow Hub scheme is cheaper, simpler and quieter than Heathrow Airport Ltd’s third runway.
“It can also be phased, so the introduction of new slots will be gradual.
“An extended runway will deliver all the economic benefits and new capacity of a third runway but at lower environmental, social and financial cost.”
A decision about a new runway was expected to be taken by David Cameron shortly after the EU referendum, if the Remain campaign had won.
Mr Lowe wrote to George Osborne, Mr Hammond’s predecessor, in June, warning that a new third runway proposed by Heathrow Airport Holdings “could require a Government guarantee from HM Treasury”.
He argued that Heathrow Hub would cost less than £10bn, which would be at least £6bn cheaper than a new third runway.
Both Heathrow schemes have offered cut-price versions of their proposals in a bid to convince ministers of their merits.