"No one here is taking any chances and waiting on any political leader to save their homes and make sure that the air here is fit to breathe," said activist Rob Jones, 26, beside plots of beans, spinach and fennel grown by camp residents.
"We cannot build any more runways and prevent catastrophic climate change," he said.
Campaigners are getting ready for a government decision on whether to extend the airport, which supporters say is vital to cope with growing demand and will boost jobs and the British economy.
If the multi-billion-pound project gets the go-ahead, however, it is likely to spark uproar among many London residents and environmental groups.
Several dozen people live in the brightly-painted camp near Heathrow, which runs on solar and wind power and where residents share meals, pitch in on building work and gardening and operate a bike-repair workshop, paying £10 (14 euros, $15) a month to stay.
Indoor areas include a shipping container turned into a dormitory, a hand-built straw mud house lined with rugs and old greenhouses fitted out with sofas and pianos where the activists gather for meetings.
A decision on the airport is expected this month but reports indicate it could be delayed by six months.
There are deep divisions within the ruling Conservative party of Prime Minister David Cameron, who himself previously opposed the expansion.
But the plan received a boost earlier this year when a report by the Airports Commission recommended it as the best way to increase capacity among London`s six international airports -- the others being City, Gatwick, Luton, Southend and Stansted.
A spokesman for Heathrow said an expansion would be "environmentally responsible" by incentivising public transport to the airport and cleaner vehicles.
But the Grow Heathrow activists promise resistance.
If the third runway is approved "there will be... civil disobedience," said Jones, speaking below treehouses the activists can climb up to if the camp is besieged.
"There is a huge amount of resolve."
Roar of planes
Grow Heathrow is one of a constellation of groups opposed to the runway and willing to take direct action to stop it.
Another, Plane Stupid, caused traffic chaos at the airport when they blocked a tunnel to its terminals with a van in a protest against the runway last month.
The project is staunchly opposed by John McDonnell, the local MP for the area and the economic spokesman for the opposition Labour party.
At a rally this month on the village green of Harmondsworth, McDonnell addressed locals from the Stop Heathrow Expansion group, many of whom could lose their homes under the plan.
As he spoke, speakers mounted on a van simulated the roar of planes passing constantly overhead.
"The noise we are hearing now is insignificant in comparison to what it would be if a third runway went ahead," McDonnell shouted through a loudspeaker.
"There is no way we will allow them to blight our homes, demolish our homes, render 10,000 potentially homeless, poison our atmosphere and also create a noise that will blight most of London."
The argument has rumbled on for years, and some residents are keen to close the chapter and receive compensation to move out of an area already blighted by traffic and airplane noise, a sprawl of hotels and a transient airport population.
But others are keen to keep their homes.
"I don`t want to move, I`m too old to move," said Brian Spink, 61, a local retiree wearing a "No Third Runway" badge.
Another is Armelle Thomas, 69, who said her husband died eight weeks after they were informed that their house would be one of those demolished under the plan.
"So for me it`s personal. Don`t talk to me about compensation. Nobody can compensate me for 46 years with my beloved husband," Thomas said, holding a photograph of her husband Tommy, a World War II veteran, in military uniform as a young man.
More about:
