The long-awaited decision on whether to build a third runway at London's Heathrow airport, which had been expected this week, has been delayed again. On 7 December Prime Minister David Cameron's office said he would give "clear direction" by the year-end. The debate over where to build a new runway in the densely populated south-east has been running for over 25 years.

An independent commission recommended Heathrow's expansion plan as the best of all the options. If Cameron agrees, his decision will end decades of political deadlock but also break a famous pledge he made in 2009 not to allow a third runway to be built at Europe's busiest hub airport. Campaign group Plane Stupid, which is opposed to any expansion of air traffic in the UK, has already sought to disrupt Heathrow's operations in protest against the proposed new runway.

"We can't have aviation expansion anywhere and stay within our carbon emissions reduction targets. Therefore it is not a case of Heathrow or Gatwick, it is a case of not at all," said spokesman and activist Cameron Kaye.

"Nine out of 10 destinations out of Heathrow are to short-haul destinations that we could be getting through to by rail and by car. If we start subsidising these industries properly we will therefore be able to create a much more environmentally-friendly and practical solution rather than expanding this carbon-intensive industry," he said.

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Three London villages near Heathrow will be virtually wiped off the map if the Heathrow expansion goes ahead. In Sipson, on land designated for the third runway, environmental campaigners have moved in and have built a self-sufficient squat camp called Grow Heathrow.

"Grow Heathrow, as well as opposing the third runway, also shows alternative ways of living. We have been sustaining around 60 people recently with solar panels and wind and using reclaimed wood for our fuels and this, juxtaposed with the wastefulness that we see around us, as a project that educates people has been very successful," said Maeve Morgan, a 24-year old student, who has been living at the camp for the past 15 months.

The land occupied by Grow Heathrow was once a derelict dumping ground before the squatters moved in February 2010. The villagers have welcomed the change of use and have dug in with the activists, to learn more about how to protest and also about eco-living.

"If we are going to live sustainably then we have got to stop the third runway, because the amount of pollution it would cause is just ridiculous really," said 24-year-old activist John McPhilips, who has lived there for a year.

Next to Sipson is the picturesque village of Harmondsworth. The proposed perimeter fence would cut through the heart of the village, leaving the remaining centuries-old cottages effectively positioned in an airport.

"If this is such a good idea then why can we not find one London mayor to support it?" said local resident Neil Keveren.

"Most of the MPs who are proposing a runway at Heathrow don't live anywhere near it. Nowhere else in Europe are so many people affected by aviation grief, that's as it stands now. Most other countries do not build their major international airports within the most densely populated areas," he added.

If Cameron does go back on his word and back Heathrow expansion, which he is expected to do, he will face a tricky time winning over lawmakers from his own Conservative party and in the opposition Labour party.

London Mayor Boris Johnson, a Conservative member of parliament, has said he would lie down in front of bulldozers to stop the third runway going ahead, while his potential successor as mayor, Zac Goldsmith, another Conservative MP, is also a prominent opponent to Heathrow's expansion. In 2010 permission was withdrawn to expand Heathrow, which objectors say already blights the lives of hundreds of thousands of Londoners. Campaigners who won that battle wonder why they are still fighting to stop the airport's expansion.

"Money talks doesn't it? The difference is that I am a firm believer in people power change and there is a lot of people that are against the third runway for different reasons," said Sipson resident and barmaid, Tracy Howard.

"My heart and soul is in Sipson. They don't want to take the other side of the airport, which is exactly the same size as Sipson and the other villages, it is used for their haulage. So that just goes to show that money talks more than people," she said.

In July, activists cut through Heathrow's perimeter fence and chained themselves on a runway, forcing some flights to be cancelled. Last month they brought chaos to road traffic around the airport by blocking the main entrance tunnel with a vehicle. Heathrow, which is operating at full capacity, argues that a new runway will improve Britain's links to other markets, adding £100 bn to the economy and more than 120,000 new jobs.