Stansted Airport will gain a second runway under the government plan

What will be the environmental impact at Heathrow and Stansted?

It will differ greatly, which is why the government has chosen Stansted over Heathrow for the first new runway in the South East.

Air pollution is the biggest issue. It is estimated the rural location of Stansted would mean just 20 people are affected by higher levels of pollutants like nitrogen dioxide, while at Heathrow the number is up to 30,000.

As for noise pollution, the Heathrow expansion will only be allowed if it does not increase the number of people affected by noise.

At Stansted up to 14,000 people will suffer from higher levels of noise.

How will Heathrow stay within the pollution limits?

Opinion varies. Many environmentalists and pollution experts believe Heathrow will never meet European pollution limits due to be introduced in 2010.

Not only will flight numbers grow steadily but so will traffic levels on surrounding roads, including Britain's busiest motorway, the M25, and the M4.

To meet the targets, BAA, which owns Heathrow, will have to introduce low-emission service vehicles, and persuade airlines to use new, cleaner aircraft engines.

Heathrow's new runway will not be built unless the company succeeds.

Is it true that air travel is subsidised more than other transport?

Not subsidised so much as freedom from taxation.

British motorists pay road tax and some of the highest duty on fuel in Europe.

Airlines do not pay any tax on their fuel, or VAT on tickets or meals.

Airports can sell products duty free, though passengers do pay �400m a year in airport taxes.

The environmental group Friends of the Earth estimates the tax breaks enjoyed by the aviation industry add up to nearly �10b a year.

The government argues that unlike roads and railways, airlines and airports operate without subsidy, and meet all the costs of building new runways and terminals.

What would happen if airlines had to pay for aircraft pollution?

Ticket prices would increase.

Charging per mile for pollution would lead to proportionately higher fares that would be most noticeable when fares are very low - such as those offered by the no-frills operators.

It is more likely, however, that ticket prices will go up to pay for new runways rather than pollution.

BAA, which owns the seven biggest airports, is not allowed to charge airlines more to land at Gatwick, say, so it can expand Heathrow and Stansted.

That would be cross-subsidisation, which is illegal.

BAA has yet to say how it will pay for expansion but higher prices at Stansted must be more likely.

Which carriers benefit most from the decision - low cost airlines or BA ?

The early extra runway at Stansted will benefit low cost rivals Ryanair and Easyjet, which are based there, as well as a host of package tour operators which use the airport.

However, these carriers say they do not want to have to pay for the construction.

At Heathrow the airlines are angry they are going to have to wait so long for more runway space, and British Airways believes Tuesday's announcement will benefit its rivals.

Why is Heathrow so important to the economy?

Because it's where most people want to fly from.

South-east airports are used by 120m of the 200m UK passengers flying each year.

London is the biggest tourist destination in the country and other popular sights, such as Oxford and Cambridge, are nearby.

Business travellers use London airports for easy access to the City, and Heathrow is an important airport for passengers transferring from long-haul international flights, to short-hop European ones.

For all these reasons, London airports are vital for the airlines, which want to maximise the facilities they offer. But Heathrow is the most important of them all.