A decision on whether to build a third runway at Heathrow Airport has been delayed until at least next summer, the government has confirmed.
Transport Secretary Patrick McLoughlin said further research on the environmental impact was needed.
A decision had previously been promised by the end of this year.
Business groups reacted with dismay at the news, with one group describing it as "gutless", but opponents welcomed the extra focus on the environment.
"The case for aviation expansion is clear - but it's vitally important we get the decision right so that it will benefit generations to come," said Mr McLoughlin.
"We will undertake more work on environmental impacts, including air quality, noise and carbon."
An independent report on airport expansion by Sir Howard Davies in July backed a plan to build a third runway at Heathrow.
But he said that the new runway should come with severe restrictions to reduce the environmental and noise effects, and did not completely rule out another runway at Gatwick or doubling an existing runway at Heathrow.
This latest development means these two other options are still on the table and opponents argue the delay weakens Heathrow's position.
Airport expansion - what are the options?
Building a third runway at Heathrow
Extending an existing runway at Heathrow
Second runway at Gatwick Airport
Gatwick described the delay as a "defining moment" in the airport expansion debate.
"We are glad that the government recognises that more work on environmental impact needs to be done," said its chief executive Stewart Wingate.
"Air quality, for example, is a public health priority and obviously the legal safeguards around it cannot be wished away," he added.
And Heathrow Hub, the group behind the proposal for extending a runway at Heathrow, said the delay "seemed sensible".
But Heathrow Airport said it had "full confidence" its plans could meet "tough environmental conditions" and would now "move into the delivery phase".
Analysis: Richard Westcott, transport correspondent
Plenty of business and union leaders will be frustrated at yet another delay to the airports' decision.
But the government says it needs more time to pick a winner.
Ministers want another six months to drill down into the impact any new runway will have on air quality and the people who'll live under the flight path.
A cynic might point out it helps them out of a political hole. Conservative MP Zac Goldsmith said he would resign and force a by-election if they picked Heathrow. But he'll be tied up with the London mayoral election, which is to be held on 5 May 2016.
Still, ministers do need to get this decision legally watertight.
Campaigners around Heathrow have already told me they plan to take any expansion plans to court, and they think the impact of a bigger airport on local traffic pollution is their best chance of winning.
Business groups reacted with anger at the delay. They argue that a lack of space at airports is damaging the economy.
"Business leaders will be tearing their hair out at the news that, yet again, a decision on expanding the UK's airport capacity has been delayed," said Simon Walker from the Institute of Directors.
"Of course this is difficult choice, which is the reason the government set up the Airports Commission to make a recommendation balancing economic needs, environmental concerns and the impact on local residents," he continued.
"We have to ask now, what was the point of the Commission if the government still fails to act?"
Meanwhile the business lobby group, the CBI said the decision was "deeply disappointing" and the British Chambers of Commerce described it as "gutless".
But London Mayor Boris Johnson, a vociferous opponent of a third runway, said the Heathrow campaign was now officially grounded.
"The wheels are falling off the Heathrow fuselage and I think that, now the government has hit the pause button, they will begin to understand with ever greater clarity that, due to the environmental impacts, the legal obstacles and the cost to the public purse, this bird will never fly," he said.
Conservative MP and London mayoral candidate Zac Goldsmith, who threatened to resign if the government picked Heathrow, said: "We can't afford more dithering over aviation capacity.
"Gatwick stands ready to deliver it sooner, at a lower public expense and without the damaging impact of Heathrow expansion."
The delay means no decision will be made before next year's London mayoral election, to be held on 5 May.
Labour's mayoral candidate, MP Sadiq Khan, told the BBC the delay was "bad news for London's businesses".
"What we should be doing is agreeing to a new runway at Gatwick Airport. Say no to Heathrow... This dithering and delay will mean problems in relation to growth," he added.
Friends of the Earth was also among those who applauded the decision, saying it was "clear you can't build a new runway and tackle London's toxic air pollution at the same time".