The transport secretary Justine Greening said on Tuesday that the government remains opposed to a third runway at Heathrow, despite some Conservative MPs calling for the idea to be reconsidered.
Here's what you need to know.
Why is a Heathrow expansion back in the news?
David Cameron is coming under increasing pressure to re-examine plans to add a third runway to Heathrow, despite previously stating that the issue was "off the table". Tim Yeo, a Conservative MP and chairman of the Commons Energy and Climate Change Select Committee, wrote in Tuesday's Telegraph that Cameron would have to decide if he was "a man or a mouse", the former by backing the plans.
However, Justine Greening, the transport secretary, has come out to defend the PM. She told the Radio 4's Today programme that a third runway was "not right" and that alternatives must be considered.
Is this a divisive issue for the government?
Yes. Though all three main parties opposed the plans in 2010, the dire economic situation has left some reconsidering the idea. In his election campaign, one of David Cameron's pledges was "No ifs, no buts, no third runway".
For him to change his mind now would cement memories of his term as one of constant U-turns, something Labour have been keen to point out in recent months. He also risks losing the support of his own party, with MPs Zac Goldsmith and Justine Greening threatening to leave the party if the plans go ahead.
The Liberal Democrats are strongly opposed to the plans and instead want to examine ideas for a new airport, though not in east London. With this in mind, continuing with the plans could further damage the already fraught coalition.
What's the history of the plans?
Plans to add a third runway to Heathrow were first proposed by Labour in 2003 and confirmed in 2009. However, when the coalition came into government in 2010, the plans were scrapped. Alternative proposals to add extra runways to Gatwick of Stansted were also thrown out. In May 2011, Boris Johnson backed plans for a new airport in the Thames estuary, dubbed "Boris Island". A consultation on the plans has been repeatedly put back and is now due to begin in autumn.
What are the arguments for a third runway?
Those who support the plans, including the British Chambers of Commerce, airlines, CBI and backbench Tory MPs such as Tim Yeo, say it will increase growth and benefit the job market. They argue that without the expansion Britain will lose out to large European "hubs" like Frankfurt and Charles de Gaulle. With Heathrow already running at close to 100% capacity, supporters of the plan say that the airport will not be able to cope with the estimated 335m passengers expected at UK airports by 2030.
And the arguments against?
Opponents of the plans, including the Liberal Democrats, Mayor Boris Johnson, Friends of the Earth and transport secretary Justine Greening, say that any benefits are outweighed by issues such as pollution levels and impact on local areas. They also say the economic argument is overstated. They believe that demand will drop as people look to alternative methods of travel. The runway would also be shorter and unable to cope with larger aircraft.
How much would a third runway cost?
BAA, who originally submitted the proposals to the coalition, said the third runway would cost around £10bn.
What are the alternatives?
A new airport could be built, however this would require a great deal of time, money and mean huge amounts of change for local infrastructures. The most popular choice at present, championed by Boris Johnson, is near the Isle of Grain in east London. A separate scheme to build an airport east of London has been put forward by architect Lord Foster. The plan comes with a £50bn price tag.
Giving another airport an extra runway is possible, but this is logistically and legally tricky. Gatwick is the best candidate, but a legal contract with the local council and the airport's previous owner means a runway could not be considered until 2019. There is also an argument to increase the capacity of regional airports, but this would mean huge changes to small areas.