Airport expansion: Which options will be cleared for take-off?
In September 2012, the government launched a review of how the UK might expand its airport capacity in the South East.
Airports had until 19 July to submit their preferred options to the Airports Commission, also known as the Davies Commission after its chairman Sir Howard Davies.
As well as the general issue of capacity, many argue that Britain needs a larger hub airport to compete with European rivals like Amsterdam Schiphol and Paris Charles de Gaulle.
So what do we know about the options being presented to the commission?
Heathrow chiefs have unveiled three options for a new third runway at the west London airport, but this plan has many opponents.
The airport's submission to the Davies Commission, which is looking at raising airport capacity, outlined a runway to the north, north-west or south-west of the existing airport.
Mayor of London Boris Johnson has put forward various plans for a new four-runway hub airport, but Heathrow's managers say each of their options is "quicker and cheaper" than plans for a rival hub airport.
Heathrow's submission said a new runway could be in place by 2029, allowing 260,000 more flights.
The airport is one of the world's busiest, handling 70 million passengers last year. More than a third transfer to other flights, making it a major hub airport.
But Heathrow operates at 98% of its capacity within a current limit of 480,000 flights per year, much closer to capacity than other major London airports and rival European hubs.
Lack of spare capacity can mean delays and cancellations can be caused by small disruptions such as bad weather.
Pros: Seen as the best short-term option to keep Britain competitive with its European rivals, as a third runway could be completed relatively quickly. The airport is a big employer and supporters cite a knock-on effect to businesses in the area.
Cons: The environmental impacts. Susan Pearson, of the Airport Watch campaign, said Heathrow would become the biggest emitter of carbon dioxide (CO2) in the country and the noise pollution would become even worse for the 725,000 already living under the flight path. Added to that, there would be a loss of homes.
Comment: "A third runway at Heathrow is the most disastrous option in terms of noise and air pollution but it would deliver far more useful flights than any other solution and we need something fast," said David Learmount, of aviation experts Flightglobal.
Mayor of London Boris Johnson and Transport for London looked at 16 options for a new hub airport in the South East, including Heathrow, Gatwick and Luton. He came down in favour of three possible locations for a four-runway hub airport to replace Heathrow.
These options are an artificial island in the Thames Estuary - dubbed "Boris Island" - a major expansion at Stansted, or an airport at the Isle of Grain in north Kent.
One version of the Isle of Grain plan, designed by the company of architect Sir Norman Foster, includes new high-speed rail links and a Thames crossing and barrier. It's estimated the cost would be £50bn.
Mr Johnson said London needed to match foreign cities which had constructed "mega airports", and said anyone who believed that was possible at Heathrow was "quite simply crackers".
His plan also includes creating a new town housing up to 250,000 people on the Heathrow site once the airport closed.
Pros: Likely to be privately funded and provide an airport hub large enough to compete with European rivals. Create thousands of new jobs to the east of London. Solve the noise problems affecting London residents.
Cons: Critics say it is likely to devastate the economy around Heathrow, and present a serious risk of bird strike to aircraft using the airport. Air traffic controllers say it is in the worst spot for traffic. Environmental campaigners say it would be a threat to rare wildlife. The cost and length of time it would take to complete are also cited as problems.
Comment: "The Thames option could provide a solution but it would also be the most costly and would take the longest. There is also a severe risk from migrating birds. Planes survive most bird strikes but the cost to the airlines is huge and occasionally, like in the Hudson River crash, they stop all the engines," said David Learmount.
Gatwick is the world's busiest single-runway airport, but relatively few passengers - about 5% - use Gatwick to transfer to other flights.
Gatwick has gone to the Davies Commission with what it says is a "robust and compelling case" for building a second runway to the south of the existing one.
Airport chief executive Stewart Wingate says the expansion would cost between £5bn and £9bn, and be financed privately.
Gatwick's submission contends that the new runway could be opened by 2025 and would have less of an environmental impact than a third runway at Heathrow.
Depending on how the second runway is used, passenger numbers at the Sussex airport could be expanded from about 34.2 million passengers a year to between 67 and 87 million.
Gatwick backs a "constellation" approach in which London would be served by three main airports, each with two runways.
Pros: If Gatwick added a second runway, fewer people would be affected by noise and pollution than the number affected by a third runway at Heathrow, which is closer to central London. Gatwick already has good transport links to London.
Cons: An existing agreement means no new runway will be built at Gatwick until 2019. The north terminal was built where the second runway would have been. A high-speed link between Gatwick and Heathrow would be a long-term, expensive project.
Comment: "It would still mean you're splitting your hub between two airports, whereas at Schiphol [airport in Amsterdam], you've got one. The market isn't there for it," said Laurie Price, an aviation consultant and former government adviser.
The current terminal at Stansted opened to passengers in 1991 and is now the UK's fourth busiest airport. Boris Johnson has included it on his list of three possible four-runway hubs.
It is the UK base of low-budget airline Ryanair, whose chief executive Michael O'Leary has called for a second runway at Stansted to help increase capacity in the South East.
A Ryanair spokesman said the government lacked a "progressive strategy" on airport expansion and said ministers should "immediately sanction" a third runway at Heathrow and second runways at both Gatwick and Stansted.
"Anti-growth nimbys cannot be allowed to oppose national infrastructure on the basis of their narrow vested interest," the spokesman added.
Pros: Fewer people would be affected by noise than at Heathrow and there is already land to build a second runway.
Cons: Poor transport infrastructure makes it a difficult trip from London, and a huge increase in passenger numbers would make this worse. Improving transport links would be costly.
Comment: "It's the wrong side of town. If the demand is north and west, why do you do it north and east?" said Laurie Price.
Birmingham airport's chief executive, Paul Kehoe, authored a report last year outlining why the government should utilise existing airports outside the South East to take the strain off Heathrow.
He said Birmingham, which currently has 9m passengers a year, could build an extra runway and take up to 70m a year passengers in the future.
Mr Kehoe said the UK needed a "network of long-haul national airports" which would help "rebalance" the economy.
Pros: Central England location. Work has already started on extending the runway for longer-haul flights, with spare capacity for more than 27 million passengers annually by 2014. Those in favour say it will help spread the economic benefits to regions outside of the capital.
Cons: Lack of interest from airlines in using Birmingham so far. Still more than an hour away from central London by train.
Comment: "Birmingham is great, but it's only got one runway. Locationally it's terrific. But there is nothing to stop any airline flying into Birmingham today. If the market isn't there, the airlines won't go there," said Laurie Price.
Opening up the RAF airport for short-haul civilian flights has been suggested as a short-term solution while a third runway at Heathrow is being built.
Currently the RAF's main base in London, it is already used for a small number of private business flights.
It used to be the key London base for British European Airways - a predecessor of present-day British Airways - handling as many as 50,000 take-offs and landings in 1957.
Pros: Located only a few miles north of Heathrow, so in a better position to serve people transferring flights than Gatwick. There is already a runway and lots of spare capacity.
Cons: The government says it needs a military airfield inside the M25, so it would be difficult to add thousands of annual civilian flights. It would add more traffic to already busy skies in the area.
Comment: "It's a ridiculous idea. It's surrounded by suburbia, and where's the infrastructure to support it?" said Flightglobal's David Learmount.
Transport Secretary Patrick McLoughlin has said the country needs to increase its aviation hub capacity to compete with international competitors. It's a view echoed by some other politicians, business leaders and aviation experts.
But there are others who say no credible case has been made to increase the UK's airport capacity.
Friends of the Earth's head of campaigns Andrew Pendleton said: "The region doesn't need more airport capacity; London already has more flights to the world's top business centres than any of its European competitors.
"The mayor [Boris Johnson] should stop pitting communities in West London against those in Essex and Kent and back transport solutions that don't wreck our planet."
Susan Pearson, communications director of Airport Watch, a group opposed to airport expansion that might harm the environment, said more effort should be made to utilise existing capacity.
"We don't have any hard evidence yet on demand," she said.
And Cait Hewitt, deputy director of the Aviation Environment Federation, said: "The idea that the UK is facing an airport capacity crisis is a myth.
"Architects seem to be looking around for work and airports want to impress their shareholders with grandiose expansion plans, but the truth is that we don't need any new runways."