By Tom Symonds
Transport correspondent, BBC News
The argument over expansion could continue for years
The debate about the expansion of Heathrow has been raging for several years - but in many ways the argument in these last days before the government's decision feels like the calm before the real storm.
If ministers say yes to the plan for a third runway and sixth terminal, there will follow a decade of argument. The two sides will fight it out in the courts, during public inquiries, and throughout construction of the runway.
On Monday, the business, union and aviation worlds brought out their big guns. Arrayed across a City of London conference room were British Airways, BMI, the CBI, London First, London Chamber of Commerce, the TUC, GMB, Unite and BAA.
They all had different ways of making the argument for expanding the airport but each said there was no alternative to it.
The BAA chief executive Colin Matthews admitted the government's decision was "not an easy one" and that the destruction of homes and farmland involved was an inevitable consequence of the construction needed.
But the airline industry argues that Britain needs to have a wide range of air travel destinations. It says that to make sure these are economically viable, it is important to attract transfer passengers - who connect from one flight to another without leaving the airport - through Heathrow.
Opponents of expansion have criticised this strategy, saying Heathrow is being expanded to benefit those who spend almost no money in Britain.
But without them, the British Airways boss Willie Walsh said there would be fewer flights to China, India, and some US cities such as Seattle.
The argument goes that more expansionist airports in European cities such as Paris and Amsterdam will happily steal these travellers. All these airports are on the right side of the English Channel for people travelling to the blossoming Asian countries, and Middle East.
One fascinating element of this debate is the way in which unlikely alliances have developed.
Big business worried about executive travel, stands alongside trade unions worried about airport jobs. The Conservative Party, which has announced it wouldn't build the third runway, is shoulder-to-shoulder with environmental activists.
On Monday, several hundred campaigners took part in a "Dinner at Domestic Departures" at Heathrow's Terminal One.
Surrounded by police, they picnicked and chanted slogans against the proposed development.
Protests are likely to continue throughout any planning process
The protesters are young, often eloquent in their opposition and, it seems, ready for a long struggle.
Gabriel Bristow, a sixth-former from north London, said: "We obviously find it difficult to get our message across but I think it's important to protest all the way from before the decision is made, and if the decision goes through I think we'll keep protesting."
Campaigners from the group Plane Stupid, whose members took part in the invasion of the runway at Stansted Airport before Christmas, stressed the feeling that their parents' generation was failing to do enough about climate change.
Aviation has taken over from roads as the subject of environmental protest.
It is likely that if runway three goes ahead the construction process will again be targeted, as it was in the 90s, with the battles at the Newbury Bypass, Twyford Down, and the M11 Link Road in East London.
But there are signs that the internet generation is finding new ways to get its message across.
Greenpeace has quietly bought a field close to the site of the third runway, right in the middle of what would be the expanded airport.
The plan is to parcel it up into tiny squares, and sell them online to people across the world.
"The airport will have to buy the land back from Eskimos and people living on remote islands," said one Greenpeace activist.
That could add up to a considerable bill for BAA, the airport's owner.
New planning laws could help speed things up, but this seems likely to be a bitter confrontation about an issue which genuinely divides people.