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The government's long-awaited decision on airport expansion is set to be announced, with Heathrow the favourite.

Transport Secretary Chris Grayling will make a statement to Parliament around lunchtime on Tuesday.

But the decision faces a long consultation before it becomes final.

A study last year, led by Sir Howard Davies, recommended a third runway at Heathrow but other options include a new runway at Gatwick or extending one of Heathrow's existing runways.

Writing in the Telegraph on Monday Sir Howard said the case for expanding Heathrow had "strengthened in recent months" and that the case was now "overwhelming".

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The issue of where to expand airport capacity in the UK has vexed politicians for years and there are strong divisions within the government.

Prime Minister Theresa May told the Commons during Prime Minister's Questions last week the subject had been "debated, discussed and speculated on for 40 years".

Heathrow is already operating at 98% capacity and Gatwick is expected to run out of space in the next few years.

Airlines and business groups favour expansion of Heathrow - Britain's busiest airport - which offers more direct connections than Gatwick and handles more freight.

But local residents and politicians are concerned about noise, traffic and pollution.

Cabinet split?

A Heathrow expansion is also strongly opposed by Education Secretary Justine Greening and Foreign Secretary Boris Johnson, who has pledged to "lie down" in front of bulldozers to stop the building.

However, neither he nor Ms Greening are members of the airports sub-committee charged with making the final decision.

As many as 60 Tory backbenchers could also vote against expansion at Heathrow, and Zac Goldsmith, Tory MP for Richmond Park and North Kingston, has vowed to resign if it goes ahead.

Media playback is unsupported on your device Media caption Reporter Divya Talwar spends 24 hours with a family living beneath the Heathrow flight path, for the BBC's Victoria Derbyshire programme

Mrs May has given ministers "exceptional and limited" freedom to criticise the government's final decision - although they will not be allowed to campaign against it - a move being seen as evidence a third runway at Heathrow will be backed.

The decision will also be subject to around a year of consultation before Parliament votes and, if approved, will only then enter the planning process.

Construction is not likely to begin until 2020 or 2021, the Airports Commission has said.

Heathrow's new runway plan

A final decision on which London airport to expand has been years in the making.

In 2009, former prime minister David Cameron pledged that there would be no new runway at Heathrow.

In July 2015, the Airports Commission chaired by Sir Howard Davies backed a new third runway at Heathrow, but did not rule out the option of expanding Gatwick.

Mr Cameron had promised a decision by the end of last year on whether to build a new runway at Heathrow.

Last week Mrs May told ministers at a cabinet meeting that a decision on increasing airport capacity in the south east of England had been "delayed for too long".

Her spokeswoman said the prime minister believed it was important to now take a decision "in the national interest".