Heathrow expansion: Cameron promises decision this year
The government will make a decision on airport expansion by the end of the year, David Cameron has said.
The PM was speaking after the Airports Commission recommended a third runway at Heathrow as its chosen solution.
Several high-profile Conservatives are opposed to Heathrow expansion, including London mayor Boris Johnson, who predicted it "won't happen".
Labour said it favoured a new Heathrow runway and claimed Mr Cameron was being "bullied by Boris".
In its final report, the commission, set up under the previous coalition government, said it would add £147bn to the economy and 70,000 jobs by 2050.
During Prime Minister's Questions, Labour's acting leader Harriet Harman said Mr Cameron had been "overruled" by Mr Johnson.
"Will he stand up for Britain's interests or will he just be bullied by Boris", she asked.
Mr Cameron, who ruled out Heathrow expansion "no ifs, no buts" in 2009, told Ms Harman there were legal reasons why the government could not announce its decision before fully digesting the report.
"If you make some precipitate decision or rule out one particular option you will actually make the decision you would like to make impossible to achieve because of judicial review," he added in a response to Conservative MP and likely 2016 mayoral candidate Zac Goldsmith.
Mr Johnson was asked on BBC Radio 4's Today programme about his pledge to lie in front of bulldozers if Heathrow expansion was approved.
He replied: "As it happens, I don't think my services as a bulldozer blocker will be required for decades, if ever."
He said a third runway would be a "precursor" for a fourth runway in the future, saying the commission's call for a law to rule a fourth one out was a "fiction".
"This is the sort of thing you could have got away with in China in the 1950s," he said, adding that the impact in terms of the environment and noise would be "so huge" that it was "not deliverable".
Mr Johnson repeated his view that an airport in the Thames estuary was the best solution.
He told BBC News the "discomfort being endured" by Londoners on the hottest day of the year "would be nothing compared to the noise pollution that will be visited on hundreds of thousands, if not millions, of innocent people and the vehicular pollution as a result of a catastrophic decision to increase runway capacity in the west of the city. That will not happen."
International Development Secretary Justine Greening said her Putney constituents would be "extremely disappointed" by the commission's recommendation of an expanded Heathrow.
On her website, she raised concerns about noise pollution, traffic, cost and the opposition from "millions of residents under the flight path".
Mr Goldsmith, the MP for Richmond Park, said he stood by his pledge to resign and trigger a by-election if Heathrow expansion was backed by the Conservative Party, but said he did not think this would happen.
"A decision in favour of Heathrow expansion is really just a decision in favour of delay and fudge," he added.
Analysis by Chris Mason, BBC political correspondent
It's cost millions of pounds and runs to hundreds of pages. But do the maps, graphs and paragraphs in the Airports Commission's final report get us any closer to diggers rolling up somewhere in the south east of England, with a few lorries full of tarmac trundling along behind?
The honest answer is a bit closer, but not much. Because whilst the commission offers weighty analysis, it's a knotty political decision in the end. And whilst the commission says Heathrow, the prime minister's "no ifs, not buts" promise not to sanction expansion there would take one heck of a lot of wriggling out of.
Just think Nick Clegg and tuition fees and what broken promises can do to you.
But here's another thought: David Cameron won't be prime minister forever. He'll be gone in less than five years. And it takes longer than that to build a runway.
Transport Secretary Patrick McLoughlin told MPs the government would look at the best way to achieve planning permission "quickly and fairly" if expansion is to go ahead.
He added that ministers would report back to Parliament in the autumn "to provide a clear direction on the government's plans".
In 2009, Mr Cameron said the Conservatives' pledge on Heathrow was made with "no ifs, no buts".
But John Holland-Kaye, Heathrow's chief executive, said the latest proposals were "an entirely different plan to the one the prime minister rejected".
"We have changed" he said, adding: "We have u-turned so that the prime minister does not need to."
BBC assistant political editor Norman Smith said Conservative MPs who favoured Heathrow had demanded cabinet ministers opposed to a third runway for constituency reasons be barred from any say in the final decision.
There are about five cabinet ministers who would be affected by a third runway, including Ms Greening, Home Secretary Theresa May, Foreign Secretary Philip Hammond and Northern Ireland Secretary Theresa Villiers.
Conservative MPs who live close to Gatwick have written to the cabinet secretary urging him to rule those cabinet ministers with a constituency interest should not be allowed a vote on the third runway.
Labour London mayoral hopeful David Lammy said the backing for Heathrow was "good news for our city", adding: "We urgently need new runway capacity."
But his rival Sadiq Khan used a New Statesman article to say expanding Heathrow would be "the wrong decision for London and for Britain", saying he would do everything in his power to prevent it if he became mayor.
But Liberal Democrat MP Tom Brake said his party was "vehemently opposed to any expansion at Heathrow, as well as at Stansted or Gatwick".
Labour MP Louise Ellman, who chairs the Commons Transport Select Committee - which has also backed Heathrow expansion - welcomed the report and said the government should not "seek excuses for further delay".
Sir Howard Davies's report said that the new runway should come with severe restrictions to reduce the environmental and noise effects.
Night flights should be banned and the government should make a Parliamentary pledge not to build a fourth runway.
"Heathrow offers the kind of long-haul connectivity - flights to emerging markets which are very important to the future of the British economy - and expanding it would allow Heathrow to offer more of those flights," Sir Howard told BBC Radio 4's Today programme.
But Gatwick chief executive Stewart Wingate said the airport was "still very much in the race".
"The commission's report makes clear that expansion at Gatwick is deliverable," he added.
A third option for extending the present runways at Heathrow was rejected.
The government has said that it will give its official response to the Commission in the autumn and it is estimated that, if given the go-ahead, any new runway would take more than a decade to build.