A final decision on whether to build a new runway at Heathrow Airport has been put off until at least next summer, the Government has confirmed.
After discussion of the controversial issue by Prime Minister David Cameron and senior Cabinet ministers, Transport Secretary Patrick McLoughlin said there was a "clear" case for airport expansion "but it's vitally important we get the decision right".
He said ministers - who had promised a decision by the end of the year - would "undertake more work on environmental impacts, including air quality, noise and carbon" and "continue work on all the shortlisted locations" - two at Heathrow and one at Gatwick.
The Davies Commission concluded in July that a third runway at Heathrow was the best option - if a string of measures were put in place to deal with noise and pollution - but left open the option of expanding capacity at Gatwick instead.
Critics say the delay will harm the economy and is a politically inspired move to avoid damaging resignations by high-profile Tories - including London mayoral candidate Zac Goldsmith, who has vowed to quit as an MP if Heathrow is approved.
The contest to succeed current City Hall incumbent Boris Johnson - himself implacably opposed to the £18.6 billion third runway project - will take place in May, with Labour candidate Sadiq Khan also against Heathrow expansion.
In a statement issued after the meeting of the Cabinet airports sub committee, the Government said it backed the commission's call for more runway capacity by 2030 but faced a "complex and challenging decision on delivering this".
"The Government will undertake a package of further work and we anticipate that it will conclude over the summer. The Government will do this quickly so that the timetable for delivering capacity set out by the Airports Commission can be met," it said.
The statement added that the Airports Commission's air quality analysis will be tested using the latest projections for nitrogen dioxide, while "the best possible package" will be developed to mitigate the impact of expansion on local people and the environment.
Mr Goldsmith claimed the delay was "good news for London".
He said: "I am absolutely delighted that, after much campaigning, the Government has heard the arguments, seen sense and will judge the options against an environmental test.
"We know that any airport expansion must meet our legally binding carbon, noise and air quality limits. There can be no doubt that in a fair contest on air quality, Heathrow will not win."
Heathrow anti-noise campaigner John Stewart, who chairs the residents' group Hacan, said the announcement demonstrates ''just how difficult it will be to build a third runway''.
''The last Labour government tried and failed. And now this Government has run into real obstacles," he said.
Heathrow chief executive John Holland-Kaye claimed he was "confident we can meet tough environmental standards".
He added: "We have support locally and nationally from politicians, business, trade unions and the aviation industry for Heathrow expansion. Let's get on and build a better future for Britain."
The shortlist of proposals being considered by the Government also features a plan to extend the existing northern runway at Heathrow to form the equivalent of two runways. This £13.5 billion proposal was put forward by Heathrow Hub, a consortium that includes former Concorde pilot Jock Lowe.
Mr Lowe said a short delay "seems sensible" given the importance of the decision, and claimed the proposal for an expanded runway is "still very much in the game".
Meanwhile Gatwick wants to build a second runway at its airport, costing £9.3 billion.
Gatwick's chief executive, Stewart Wingate, said the delay was a "defining moment in the expansion debate" and claimed Heathrow's supporters must now realise that a third runway will never go ahead "as the environmental hurdles are just too high".
He went on: "We have always maintained that this decision is about balancing the economy and the environment. Expansion at Gatwick would give the country the economic benefit it needs at a dramatically lower environmental cost."