A cross-party commission will be set up to try to find consensus on the deeply divisive issue of airport capacity in the south east of England, the Prime Minister signalled today.
David Cameron told the House of Commons that he would not break his manifesto pledge ruling out a third runway at Heathrow during this Parliament, but added that a decision was needed on the fraught issue of air access to London amid growing business clamour for more flights.
London Mayor Boris Johnson - a fierce opponent of a third runway - today stepped up pressure on the Prime Minister, urging him to rule out expansion at Heathrow beyond the 2015 election.
Labour said it had been calling for a cross-party commission for a year, and accused Mr Cameron of "weakness and dither" over the issue. Labour leader Ed Miliband is "sceptical" about the case for a third runway, but is ready to look at the Government's terms of reference for the proposed talks, said a source.
Speculation over a possible U-turn on the coalition agreement pledge not to add extra runways at the west London airport was sparked by Mr Cameron's decision to move Justice Greening - whose Putney constituency lies under Heathrow flightpaths - away from the job of Transport Secretary in yesterday's reshuffle.
But asked at Prime Minister's Questions in the Commons today whether he was preparing the ground for a policy shift, Mr Cameron said: "While I believe we need to establish a form of review that will bring parties together and make a decision about airport capacity, I will not be breaking my manifesto pledge."
An announcement is expected next week on a commission to include all parties, though it was unclear whether it will be asked to make its recommendations before the 2015 election.
A Labour source said: "We called for cross-party talks a year ago, but we need to get on with it. This is another sign of dithering and weakness from the Prime Minister.
"We recognise the need for increased capacity, but we are sceptical about a third runway at Heathrow. There are deep environmental considerations and we want to minimise the impact on local communities."
A third runway was approved by the previous Labour government, but the plan was scrapped by the incoming coalition Government in 2010, after both Conservatives and Liberal Democrats made ditching it a manifesto pledge.
The Tory manifesto stated: "Our goal is to make Heathrow airport better, not bigger. We will stop the third runway and instead link Heathrow directly to our high speed rail network, providing an alternative to thousands of flights. In addition, we will block plans for second runways at Stansted and Gatwick."
Mr Johnson said he feared there was a "stealthy U-turn" under way and vowed to fight the party leadership if it backed a third runway.
He told BBC Radio 4's World at One: "What I worry about is that we are now seeing a stealthy U-turn being carried out which I don't think is in the interests of London or indeed of the country as a whole because in the end you can expand Heathrow and you can put in a runway, and actually it will be a short runway, but you have to come back in ten years time and do another."
Asked if he would lead a campaign against expansion at Heathrow he replied: "You bet I will, yes."
Mr Johnson said the possibility of a third runway was "being left open" and claimed the commission would not resolve the problem.
"It's just a fudge, it's just a fudgearama and it's just an excuse for a delay - there's almost three years to run until 2015. If such a commission were not to report until after the next election we'd have lost a huge amount of time. I don't think British business would be remotely satisfied with that answer."
Mr Johnson said he was under the impression Chancellor George Osborne supported a third runway. He insisted he remained a "keen supporter" of the Government but warned it was time for the leadership to "level with Londoners".
Tory MP and environmental campaigner Zac Goldsmith, who has threatened to quit his Richmond Park seat and force a by-election over the third runway, told BBC Radio 4's Today programme: "We haven't just lost Justine Greening from this department, we have also seen Theresa Villiers moved out, both of whom were absolutely rock solid on this issue in terms of defending what is still the Government's line officially.
"I think their movement out of the Department of Transport shows the Government is at least trying to open the door to the possibility of a third runway."
Opponents of a third runway suspect that Mr Cameron may be preparing the ground to change his party's policy for the 2015 general election manifesto.
Mr Cameron's official spokesman told a regular Westminster media briefing: "Our position has not changed.
"There is a coalition agreement that has a commitment that this Government will not build a new runway at Heathrow.
"We have said that we recognise there is an issue about airport capacity and we will shortly announce a process to look at that."
Liberal Democrat leader Nick Clegg insisted a third runway "will not happen" under the coalition.
"My party's manifesto is against a third runway and the coalition agreement could not have been clearer: we have ruled out giving the go-ahead to a runway during this Parliament. It will not happen during this Parliament," said the Deputy Prime Minister during a visit to an east London school.
Asked if the Lib Dems might be prepared to back a third runway in their own manifesto, he said: "What other parties want to do in their own manifestos for the general election of 2015 is entirely up to them. The Conservatives can change their stance in whatever way they want.
"But I am persuaded and my colleagues are persuaded that, yes we need to look dispassionately at the issue of how we ensure there are proper links between this country and growing economies, but you can't do that at any old environmental cost and there are millions of people who are affected by planes flying in and out of Heathrow every day.
"There might be very good other viable alternatives that need to be explored."
Labour's 2010 manifesto supported a third runway at Heathrow, but Mr Miliband later distanced himself from the policy, saying he had had "some very heated arguments" with Gordon Brown over the decision and even considered resigning from the government.
Senior Conservative backbencher Tim Yeo, who last week called on Mr Cameron to show whether he is "a man or a mouse" on airport capacity, said the third runway had become a "totemic" issue.
Mr Yeo told BBC News: "The business community now is almost unanimous in saying this is an urgent issue. We need more runway capacity in the South East in the 2020s, not in the 2030s.
"We've made a very good start actually by moving Justine Greening, who was a good transport secretary but had a difficult constituency issue to address.
"If we made this decision now, it would send a really encouraging signal to all those people who are investing here."
Downing Street declined to confirm widespread reports that economist Sir Howard Davies had been asked to chair the commission.
A former Treasury adviser and ex-deputy governor of the Bank of England, he has held a string of high-profile posts including spells heading the CBI and the Financial Services Authority.