By Tom Symonds BBC transport correspondent

It has to be one of the government's most difficult decisions.

We all want to fly, but most people don't want an airport runway anywhere near them.

A major consultation exercise is underway to try and solve the problem.

Government planners have been traipsing around every airport, or airport-sized patch of land in the country, to try and decide where the UK's next runway should go.

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The country's biggest airport operator, BAA, has now stepped into the fray.

Asked by the government to give its views, the company has come back with a highly controversial answer.

BAA has suggested adding one or two new runways to Stansted Airport.

There could also be another one at Gatwick.

But it's the proposals for a third runway at Heathrow that will most raise the hackles of environmentalists.

New view

The problem is that four years ago, when it was trying to get permission for a fifth terminal at Heathrow, BAA made a promise.

A BAA press release from 12 March 1999 said: "It is the company's view that the local communities around Heathrow should be given assurances.

We can't afford to see in aviation the problems we have today on our railways Mike Clasper BAA chief executive

"BAA would urge the government to rule out any additional runway at Heathrow, and BAA would support a recommendation by the inquiry inspector in his report that the government should rule it out.

"Indeed BAA invites the inspector to make such a recommendation."

But now things have changed.

BAA's response to the government consultation, the Future Development of Air Transport in the UK, included: "In our view, taking everything we know into account, a scheme for a new 2,000-metre runway at Heathrow should be included in a shortlist of four possible sites."

'Broken promise'

BAA defends its decision by pointing out that it was asked by the government to consider the proposals and that ministers have the final decision.

Opponents say it is a U-turn, the breaking of a promise.

But runway number three at Heathrow would enable the airport to handle 26 million more passengers a year.

That would help considerably in coping with the predicted tripling in demand for air travel over the next 30 years.

Heathrow is also where the airlines want to land.

But what are the alternatives? BAA is calling for up to three new runways to be built.

Legal agreement

Stansted will almost certainly get one extra runway, and possibly two.

Adding a new runway to Gatwick could prove difficult. The government would have to scrap a 1979 legal agreement prohibiting expansion before 2019.

Environmentalists have a difference solution. Get rid of the tax breaks the airlines enjoy, they say, and higher ticket prices will put off those who really don't need to travel.

But BAA believes not building runways would cause significant damage to Britain's economic position in the world.

"We can't afford to see in aviation the problems we have today on our railways," the company's current chief executive, Mike Clasper has said.

So Heathrow will be where the confrontation between the aviation industry and its opponents will take place.

Like many flights from the airport, it could be long and turbulent.