The poachers perch on the rough platforms they have built in the trees about 15 feet above the forest floor, waiting patiently for the tiger to come. They have been searching the forests of India's Ranthambhore reserve for days, following the pug marks and other tell-tale signs. When they found the fresh kill, they knew it would only be a matter of time before the tiger returned to eat. Working quickly, they placed their traps on the path, scattering small stones across the dry sandy soil, knowing that tigers hate to walk on them and will pick their way around if they can.

The tiger pads forward, guided by the stones into the trap, which springs shut with a snap. The poachers have fashioned the device from old car suspension plates; there are no teeth, because a damaged pelt will fetch less money. In pain and desperate to free itself, the tiger thrashes around. Another foot catches in another trap, then a third.

The poachers watch to make sure it cannot free itself, then edge down to the ground, still cautious, because a male Bengal tiger can weigh up to 500lb (227kg) and a female 300lb (136kg) and a single blow from those claws could kill a man. One man carries a bamboo stick into which he has poured molten lead to give it more weight. The other has a spear on the end of a 10ft pole. As the tiger opens its mouth, the poacher with the spear lunges forward, stabbing between its open jaws. As the blood starts to flow, he stabs again and again. His colleague smashes the tiger over the head with the stick.

When it is over, they draw their heavy iron knives and set to work to skin it. They leave the paws intact; they are too fiddly to waste time on out in the open. Half an hour later, they are gone, melting away unchallenged into the jungle, once more eluding the forest guards.

It is always the same, says Dharmendra Khandal, toying with a heavy iron skinning knife as he recounts the story. Khandal is sitting in the offices of Tiger Watch on the edge of the national park, one of the most popular tiger reserves in India. He spreads his palms in frustration. The government's forestry department is always the last to act, he says, though it is its job to protect the tigers.

Tiger Watch was established in Rajasthan 12 years ago as an independent, privately funded organisation trying to stem the decline of the wildlife population in the Ranthambhore reserve. In the last five years, it has helped police arrest 47 alleged poachers from the Moghiya tribe, many in possession of tiger skins and other body parts, guns and traps. By their own admission, the poachers have killed more than 20 tigers. Yet in the same period, the authorities in the park did not record a single incidence of poaching. Something does not add up.

At the turn of the last century, there were an estimated 45,000 tigers living wild in India's forests. By the time hunting was banned in 1972, their numbers were down to 2,000. In January, the World Wildlife Fund placed the animal in its list of 10 key creatures facing extinction, warning that while counting tigers is notoriously difficult, there might only be 3,200 left in the wild worldwide. The WWF has just launched a Year of the Tiger campaign to coincide with the start of the Chinese year of the tiger. The organisation is working with world leaders towards the goal of doubling wild tiger numbers by 2022 and there will be a summit in Vladivostok in September attended by the heads of government from the tiger range countries. Nowhere will the challenge be greater than in India, home to that symbol of the country, the royal Bengal tiger.

The Indian government claims 1,411 tigers are still alive inside its borders. Few experts believe this figure. When a tiger skin can sell for $20,000 in neighbouring China, poaching remains a serious problem. Last year was the worst since 2002 for tiger deaths and even India's Ministry of Environment and Forests concedes that its way of counting tigers is so vague that there may be as few as 1,165. Environment minister Jairam Ramesh now admits the figure of 1,411 was "an exaggeration". Either deliberately, to hide the true scale of the animal's decline, or accidentally, through flawed methodology, it is now clear that the numbers are wrong. Some conservationists believe the true number of tigers left in India may be little more than half the official tally and that at the present rate of decline, the tiger will cease to be a viable wild species in India within as little as five years. If poaching and habitat loss continue unabated, those reserves that still have tigers will be little more than open-air zoos. According to the ministry, there are 16 reserves (just under half the total) where there may be no tigers at all or where the tiger is in danger of becoming extinct. Part of the problem is that the presence of tigers is a matter of pride, both for states and individual reserves. No one wants to admit that their tigers have been poached. And still the forests are vanishing as India's burgeoning population places increasing demands on limited space.

Ranthambhore is one of the better parks, one of the few places visitors have a realistic chance of seeing a tiger in the wild. Even here, the number of tigers left is in dispute.

According to Khandal, Tiger Watch's field biologist, there are two schools of poachers: the professionals who tend to come in from Haryana and use only leg traps and the local Moghiya tribe who fire on the tiger from close range with homemade guns. "The Moghiyas are criminals," says Khandal. "They are one of the most brutal communities in India. A month ago, some of them cut off a woman's feet just to steal her ankle ornaments. She bled to death."

In an attempt to stem the tide, Tiger Watch has started working with the Moghiya, hiring informants for 3,500 rupees (£50) a month, while setting some of the women to work producing handicrafts and providing education for their children.

"It's a risky job," says Khandal. "We have four regular paid informants from this community and we give them money in return for information. The community knows who the informants are. Some of them are resisting but there are cracks in the society now. Some of them are asking why they should live in such a primitive state."

Kesra, 45, is one of the former Moghiya poachers who have been turned. By his own admission, he has killed at least five tigers. He describes roaming the forests looking for pug marks and then taking up position in the trees to wait for the tiger to come, working at night and returning in the morning to skin the tigers. He says they never had any trouble with the forest guards, a common refrain. He was arrested as a result of a Tiger Watch raid and is awaiting trial. He insists he is now reformed. "I never had much education. My forefathers were doing hunting, but now times have changed. We are different people," he says.

His wife, Sanwali, also 45, earns about 3,000 rupees a month from making baskets for Tiger Watch. They have five sons and two daughters to support. She says that, like the tigers, they have become the hunted.

"We are not willing to live in an atmosphere where the police are always coming after us," she says. "We had to move from here to there. Our forefathers were involved in poaching, but we don't want to be involved in this trade any more."

It is a view echoed by 26-year-old Asanti. Her family are notorious tiger poachers and she is married to a former small-time poacher, Deshraj, 30. The couple, who married when Asanti was 10, have an eight-year-old daughter, Puja, and say they don't want her to grow up like they did, shunned by the rest of society. They provide information on what is happening in the tribe and in return receive money and a chance to start afresh.

"We want our children to be educated. We want to learn more. We want a regular source of income," says Asanti. "Hunting is not a regular source of income. Times have changed and our community is scattered. Now we want to live respectably."

Tiger Watch's approach is clearly having an effect, but that has not been enough to save it from the wrath of the authorities whose indolence it has exposed. Not long after the group revealed that poaching had reduced tiger numbers in Ranthambhore to just 18 in 2004, officials turned up at the office of its founder, Fateh Singh Rathore, and demolished it. His daughter's shop and their restaurant were also flattened, ostensibly for operating without the correct permissions, though others in a similar situation were left untouched. It was a warning.

Fateh Singh is now 75. He was the government's field director at Ranthambhore from 1977 to 1996 and is regarded as one of India's foremost tiger experts. Sitting in his rebuilt office, he picks up a newspaper and stares at the large WWF advert on the front page, with its warning that there are only 1,411 tigers left in India. He shakes his head; the true figure is probably closer to 800, he says. "They are always saying that the numbers are on the increase, but there is no proper scientific research. They are lying to save their skins. If they have a problem they should declare it. The authorities like only praise."

He doubts there are more than 20 tigers left in Ranthambhore.

"The field directors are responsible. They are not trying. They are too busy showing VIPs around to spend time on protection. All the popular parks are suffering from the same disease. They know they are posted for two years and then they will go somewhere else. No one is being punished for the tigers that are being lost."

Still, he says, while there are still some tigers, there is a chance. "I am still optimistic because I feel the tiger has a lust for life. It can survive if it gets protection, but you have to be very strict if you want to protect the tiger."

The system, however, is simply not geared up to deter the hunters. There were 72 arrests for tiger poaching in India last year, but the only two convictions were for cases dating back more than 10 years.

It is hardly a deterrent. Tiger poaching is a lucrative business for some – though not necessarily the poachers, who may have to share the 100,000 rupees (£1,450) they will get for one tiger between 10 gang members – and there are plenty of people with an interest in turning a blind eye.

When Tiger Watch and the Rajasthan police went after one of the biggest poachers in the region, Devi Singh, they had to sneak across the state border into Madhya Pradesh to snatch him from his village without alerting the local authorities because, Khandal explains, had they revealed their true intentions, someone would have tipped Singh off. When they got him back to Rajasthan, Singh confessed to killing five tigers in the park, in a period when no poaching was officially recorded.

The last full tiger census in India – which claimed 3,642 tigers – was carried out in 2001, based largely on pug marks, a hopelessly unreliable method of counting. Satya Prakash Yadav, deputy inspector general of the National Tiger Conservation Authority in Delhi, admits it was "seriously flawed" and "not scientifically correct". For the latest study, he says, officials switched methods, using a mixture of camera trap results and a survey of the habitat and prey base to produce an estimate of how many tigers might conceivably have survived. But he admits that problems remain. (Yadav did not have any figures for the number of tigers actually recorded in the camera traps. There are no data for this in the latest report and repeated requests for the vital statistic drew a blank.)

Many of those reserves are already on the brink. The greatest threat to the safety of the park officials comes from the Naxalites, Maoist guerrillas who have been described as the greatest threat to India's internal security. They have seized control of vast swaths of the country, ostensibly in the name of tribal peoples who they claim have been oppressed. They have a particular loathing for forestry officials, who they regard as the stick with which the state beats the tribals, extracting money and goods from them in return for the use of the forests on which they rely for their livelihoods. At least six of the parks are overrun by Naxalites and are inaccessible to the forest department. There is simply no way of knowing how many tigers remain there and certainly no way to install camera traps.

It is hardly surprising that the latest update lists 16 of the 37 reserves as being in a "poor" state. It is possible, Yadav concedes, that there are no tigers there.

"We have classified some reserves as poor where there is no population of tiger or where the tiger may go extinct. Despite our various milestone initiatives, the situation may go out of control in certain tiger reserves."

Simlipal reserve, in Orissa – the fourth largest in India – provides an insight into just how problematic the official figures are. A 2004 report, based on pug marks, claimed that there were 101 tigers in the reserve. Last year, India's environment minister, Jairam Ramesh, conceded that 40 tigers had been poached from the reserve over the previous five years, but insisted there were still 61 tigers alive and well in Simlipal alone. Yet the government's own figures claim that there are only 45 tigers in the whole of Orissa state, which also includes those in the Satkosia reserve. Again, something does not add up.

Then there is Panna. The latest report claimed that there were approximately 24 tigers in the 974sq km reserve. Last year, it was found that there were none. And this was three years after the government had announced a complete overhaul of the system, after the Sariska reserve was also found to be empty.

Luckily for the tiger, complacency is not endemic. In the Periyar tiger reserve in Kerala, a small group of women has been mounting their own fightback. Every day, members of the Vasanta Sena (Green Army) venture unpaid and unarmed into the forest, in search of poachers. There are 76 of them, living around the edges of the park, mostly from poor families, each taking one day a week off from jobs and looking after their homes to seek out intruders. One aim is to stop the destruction of the tigers' habitat, the forest itself. The sandalwoods are prized by illegal loggers for their oil, which is used in medicines and cosmetics. One kilogram of the wood can fetch 5,000 rupees.

The forest is lush and green, a gentle breeze rustling the leaves of the sandalwoods and the swaying stands of giant bamboos arcing overhead. A small stream runs beneath a roughly made wooden bridge. The women pick their way among the trees. At the front is Gracykutty, 39. She is married to a mason and has two daughters. She has been doing this for seven years.

"Here we breathe the best air in the world and we are dedicated to protecting it," she says. "I think if there is only one tiger left in the world in the end, it will be here."

Her colleague Jiji, 35, says they know that if the forest goes, so too will the tiger, destroying the tourist industry on which their economy depends.

"We keep a look out for trees that have been cut or signs that people have been in the forest. It is important because if the forest is cut then there is less space for the animals and if the forest goes and the tigers go then it will be terrible for everyone who lives around here. We understand this and that is why we are doing this. It is not just for ourselves, it is for our children too, so they can enjoy the forest like we do."

How many tigers remain in Periyar is a matter of conjecture. Sanjayan, the range officer, says the park has about 34 tigers, maybe 36. He says camera traps have identified 24 and the rest have been calculated using the unreliable pug mark method. But his boss, Bastian Joseph, the assistant field director, cites the official figure of 46 tigers.

Many conservationists fear that without drastic action, the only place the tiger will soon be found in India is in its zoos.

Inside the royal Bengal tiger pen at the Arignar Anna Zoological Park in Chennai, Nagammal, the woman who looks after the tigers, spins a metal wheel on the wall to slide open the internal cage door. Padma, the zoo's 15-year-old female, has been growing increasingly restless. Now she pads through the open door, lets out a roar and launches herself at the thick metal grille with a shuddering crash. She lands and turns away, pacing around the cage before repeating her assault several times, roaring her displeasure. Eventually, she settles on the floor and sits watching warily, emitting a low growl. Up close, it is easy to understand why the poachers are so keen to make sure their prey is securely trapped before they approach.

The zoo's director, PL Ananthasamy, argues that the answer to the tiger's decline lies in a captive breeding programme. "The basic game is conservation and in due course of time to take these species back to their home and release them," he says.

Tigers breed well in captivity, but releasing them into the wild is another matter entirely and most experts agree that it is fraught with difficulties, which may explain why there do not appear to be any examples of successful reintroduction of tigers.

Ananthasamy disagrees: "It is possible to release captive bred animals. We must do it gradually and ensure that the animal can survive by itself. We have not yet reached the stage where the tiger cannot breed in the wild, but the pressure is such around the sanctuaries that the numbers are coming down. There is enough prey base for the animals to survive, but the problem is the encroachers and poaching."

Aditya Singh, 43, conservationist and tiger expert, worries that time is running out. Singh runs a lodge on the edge of the Ranthambhore reserve park and spends much of his time inside the park. "I think the numbers have gone down. I think there are about 1,000 now," he says.

What will finish off the tiger as a viable species, he says, is the final destruction of the remaining corridors of forest that link the parks together. "There are still connections between the reserves, but in five years they won't be there. I think the tigers have five years. They will stay in isolated pockets, but they will have reached an evolutionary dead end.

"There is a view here that the forest belongs to the foreigners. For an average villager living outside the park they don't see it as an asset. They used to be able to go in for wood, but now they cannot. The problems for the tiger are poverty, illiteracy and overpopulation. The big problems that India has are the problems the tiger has."