A long-awaited report into airport expansion is set to spark fresh controversy today among politicians, protest groups and environmental campaigners.
The Airports Commission has spent three years examining the merits of new runways and extra capacity, with the choice whittled down to three options - a third runway at Heathrow, extending an existing one, or building a second runway at Gatwick.
The final recommendation has remained a closely guarded secret, keeping supporters and opponents of the rival airports guessing about the outcome.
The Government is not expected to take any immediate action on Sir Howard Davies's report and is likely to announce that ministers will study the recommendation.
There have been suggestions that the Government could delay its final response until after next May's London mayoral election, which will be dominated by the airport runway debate, especially if Sir Howard recommends one of the Heathrow schemes.
Conservative MP Zac Goldsmith, bidding to win his party's nomination for the mayoral election, is opposed to Heathrow expansion, while the airport's future has split Labour's leading candidates.
There has been unconfirmed speculation that the report is likely to recommend expansion at Heathrow, but only if the airport meets stringent conditions on air quality, noise pollution and carbon emissions.
Gatwick has argued its expansion would be cheaper and more environmentally viable.
Local opponents of Heathrow expansion say hundreds of thousands more people would suffer from noise and increased traffic congestion.
They say it would also worsen air pollution, from plane fumes and traffic on the M4 and M25 motorways, an issue the Government has already been ordered by the Supreme Court to take immediate action on across the country.
If Heathrow is given the go-ahead by the commission, it will be "far from the end of the story", opponents warn.
John Stewart, chairman of residents' group Hacan, said: "Even if Davies recommends a third runway, there is no guarantee the Government will go for it as there are so many 'big beasts' in the Cabinet opposed to it.
"A Davies recommendation for Gatwick could mean curtains for Heathrow as the Government is likely to see it as the more politically deliverable option," he added.
Local residents campaigning against a new runway at Gatwick say expansion there will cause increased noise, pollution, new flightpaths and traffic congestion, hitting the area's heritage and natural landscapes.
Green groups are fighting against a new runway at either airport, warning that expansion will undermine efforts to cut emissions in the fight against climate change.
Friends of the Earth's head of campaigns Andrew Pendleton said: "It's simply not credible for the Government to build a new runway in the South East and still claim to be serious about tackling climate change.
"Airport expansion will also have huge impacts on the local community, noise levels and air quality.
"We can't preach to the world about stopping catastrophic climate change on the one hand and send aviation emissions soaring on the other."
Greenpeace UK executive director John Sauven said: "The solution is not to build more tarmac strips but to manage demand.
"And one way to do this is to make flight taxes fairer so that the burden is shifted from families flying on an annual holiday on to frequent fliers who are driving the expansion and pushing up carbon emissions."
The commission received more than 50,000 responses during a 12-week consultation on the three proposals which ended in February.