A decision on whether or not the so-called "Boris island" option of a new Thames Estuary airport is to be added to an official airport expansion shortlist will be made in autumn 2014, the head of the Whitehall-appointed Airports Commission said today.
Chaired by former Financial Services Authority chief Sir Howard Davies, the commission has already shortlisted Heathrow and Gatwick Airports as possible sites for a new runway by 2030.
The estuary airport plan, favoured by London Mayor Boris Johnson, failed to make the shortlist in last month's interim report but the commission promised to look further at the scheme.
Today, announcing the future work of the commission in an "appraisal framework" timetable, Sir Howard said a decision on whether to add the estuary airport option to the shortlist would be taken in "early autumn".
Addressing an aviation conference in London, Sir Howard said the estuary scheme, costing as much as £112 billion, presented "many challenges and risks", adding that the cost looked "rather daunting".
He also said that, in studying the estuary option, the commission would have to "look at the implication of closing Heathrow and London City Airports".
Mr Johnson is bitterly opposed to a new, third runway at Heathrow and favours, instead, a brand-new airport in the Thames Estuary .
Earlier this week it was revealed that Mr Johnson had written to Sir Howard asking him not to sideline the estuary plan.
Launching the commission's interim report last month, Sir Howard said the estuary airport plan could cost as much as £112 billion - much more than either the Heathrow or Gatwick options.
He also said the construction challenge in building a new estuary airport would be "massive".
An extra, third runway at Heathrow was given the go-ahead by the Labour government in 2009, but scrapped by the coalition Government when it took power in May 2010.
In a letter to Transport Secretary Patrick McLoughlin, the 2M group comprising a number of local councils said that Sir Howard had shortlisted potential sites for new runways before assessing their environmental impacts or delivery costs.
Ray Puddifoot, leader of Hillingdon Council in west London and a 2M spokesman, said: "In any scenario, expanding Heathrow will have the most severe noise and air quality impacts. Its location guarantees this. It's astonishing that Heathrow can be shortlisted without understanding the true costs of mitigating the environmental and community damage that expansion would bring."
Ravi Govindia, leader of Wandsworth Council in south west London, said: "Sir Howard is proceeding as if people's attitudes to noise hadn't changed since the early 1980s. We have urged the commission to order a new social study now so that the noise standards are based on credible evidence of community impact."
Yesterday the London Assembly passed a motion reaffirming its opposition to the expansion of Heathrow and suggesting that more use could be made of spare capacity at other airports serving south-east England.
Assembly member Caroline Pidgeon, who proposed the motion, said: "I am really disappointed to see Heathrow back on the table."
Deputy Prime Minister Nick Clegg declined to "categorically rule out" a third runway at Heathrow, but said he would not back it if it was shown to increase noise pollution or air pollution.
Richmond Park Tory MP Zac Goldsmith, a vociferous opponent of Heathrow expansion, ambushed the Lib Dem leader on his weekly phone-in show on LBC 97.3 radio to challenge him definitively to rule out support for a third runway, regardless of the recommendations of the Davies Commission.
Liberal Democrats have a long-standing position of rejecting a third runway, but Mr Goldsmith said there was now "a lot of confusion" over their stance because they had given their support to the commission's work.
Mr Clegg said: "The reason we have always ruled out Heathrow expansion is the environmental impact, because of the impact on air pollution and noise pollution, particularly on people living under the flight paths.
"Clearly any suggestion which leads to expansion of Heathrow which increases the existing levels of air pollution and noise pollution, entirely consistent with what we have said for years, we will reject."
But Mr Clegg said he would consider the findings of the Davies Report and base his decision on the evidence.
"I am a person who tries to be led by the evidence. I will look at it," he said. "But I don't think I can be any more unambiguous by saying the reason we have always opposed Heathrow expansion is because of the effect on the environment, noise pollution and air pollution. Anything that does that in a bad way is something we are clearly going to reject."
At today's conference Sir Howard said that in considering options for airport expansion it was necessary to "ensure all decisions are sensibly and soundly based and not vulnerable to legal challenge".
On the estuary airport option, the commission will look at such areas as surface transport, environmental implications, socio-economic factors and operational feasibility.
Sir Howard said that with the estuary option there was the "simple challenge of distance", with the schemes being some way from central London . He added: "The economic implications are quite serious as well, such as what would be the effect of closing Heathrow and London City airports."
Publishing the commission"s "way-forward" plans Sir Howard said interested parties would have until February 28 to comment and then the promoters of the Gatwick and Heathrow schemes would be invited to submit more detailed plans of their proposals.
Sir Howard said his team would look into the Heathrow and Gatwick plans in more detail than was possible during work to produce the December 2013 interim report where around 50 options had to be considered.
The plan was to be able to make a final recommendation in the summer of 2015, "that will allow a new government to hit the ground running".
Sir Howard said Heathrow and Gatwick were the most credible locations for a new runway by 2030, adding that "which of them is more credible will depend in part on how demand develops".
He added that although the commission had not found the claims on the shortlist from Stansted and Birmingham to be "very persuasive", it was nevertheless vital that these and regional airports grew as the chosen new runway would not be operational until the late 2020s.