When the Jarawa tribe of hunter-gatherers began to emerge in ones and twos from the dense rainforests of the Andaman islands in 1997, it seemed that these mysterious, handsome people only wanted to take a brief look at the modern world and would soon return to the trees.

But in the months that followed, shy Jarawa youths slowly gained in confidence and could be found hanging out on the side of a road recently built through their land. Then they started to stop cars and buses going by, and to beg for food. They even began to board ferries to travel between the islands.

No one knows why these people - one of the original tribes of the Andaman and Nicobar islands, a remote archipelago in the Indian Ocean - decided to leave the forest at that time. Twelve years on they have become a tourist attraction. Local companies take people to gawp at and photograph them as if they are animals in a zoo. Some throw sweets and biscuits to them, or offer them lifts and money.

The majority of the Jarawa, thought to number about 250 people, remain deep in the forests, but some have learned bits of Hindi and regularly visit the port, the hospital or market place, says Sophie Grig, a researcher at human rights organisation Survival International who has visited the Andamans several times. One or two Jarawa children have reportedly gone to a school and asked for education.

Integration has been partial and more or less at the Jarawa's own pace and volition. But now comes a threat that some anthropologists say could lead to the extinction of a tribe that has lived in isolation for millennia.

Barefoot India, a major Indian travel company, has just won a high court case that will allow it to build an eco-resort at Collipur, close to the designated Jarawa reserve. Other hotels are expected to follow.

Barefoot, which already has an Andamans resort on Havelock Island, plans to bring in thousands of tourists a year from Europe to scuba dive and to explore the remote islands now becoming popular as one of Asia's least visited beach destinations.

But Survival fears that the increased contact with tourists will inevitably expose the tribe fully to diseases and cultures that they will never be able to cope with. "Evidence from around the world is that isolated tribal peoples have little or no immunity against diseases like flu and measles, and it is certain that the more contact there is between the tribe and tourists that diseases will devastate them," says Grig. "It's not unusual for 50% or more of a population to die soon after contact. One epidemic can lead to severe depression, alcohol abuse, dependency and even suicide.

"It's incredibly dangerous. Why does Barefoot have to go there? There are plenty of other places."

Grig continues: "The biggest concern is disease. The Jarawa are incredibly vulnerable. Then there's alcohol. People in this situation are vulnerable to addiction and dependency."

A spokesman for Barefoot says: "Barefoot would not countenance any exploitation of Jarawa for tourism purposes from any of its guests, and most certainly will not attempt to do so itself. The Jarawa have no access to the resort's land, which is more than three kilometres away. [Far from threatening the tribe] Barefoot has had an extremely positive impact on the tribal interplay with the villagers in this area."

There are perhaps 100 indigenous communities around the world that have chosen to live in complete isolation, but the frontiers of tourism are being pushed ever forward by cheap flights and an appetite for extreme ethno-tourism fuelled by the natural instinct of man to be curious about other people - and by shows such as Bruce Parry's documentary series Tribe.

The Jarawa are peculiarly at risk because they live so close to a holiday resort, but dozens of other extremely remote groups are also in danger. In the West Papua province of Indonesia, US expatriate Kelly Woolford of Papua Adventures offers - for $7,000-$10,000 - to take tourists and camera crews deep into the forests of the Mamberamo and Baliem valleys, where he says they are quite likely to meet "stone age" tribes.

Papua Adventures does not guarantee "encounters", but its "first contact" trek is advertised as a "full-on exploration" in areas where previously contact-free tribes are known to live.

Groups regularly stumble across tribespeople who appear to threaten them with bows and arrows, but who then disappear. Anthropologists and others who have seen photographs have accused Woolford of setting up these encounters, but he insists that the meetings are all by chance.

"Tourism can be a useful source of income, but most people would say it's pretty bad news for the local people," says anthropologist David Turton.

Turton has spent 40 years among the semi-nomadic Mursi in the Omo valley in southern Ethiopia, where some women have had their lower lip pierced and stretched so that a clay plate can be inserted. With the prospect of a giant dam flooding much of their lands, the tribe has enough problems, but it has been exploited by tourism now for 20 years.

Tour companies have presented the Mursi as the most primitive and wild people and the Mursi are fully aware they are being singled out as savages. The tourists arrive in four-wheel drive vehicles and the Mursi gather around them, asking for money in return for being photographed.

Turton has asked the Mursi what they think of these people, who only seem to want their photographs. He recorded this conversation in 1991:

Bio-iton-giga: "Why do they do it? Do they want us to become their children, or what? What do they want the photographs for?"

Turton: "They come because they see you as different and strange people. They go back home and tell their friends that they've been on a long trip, to Mursiland. They say, 'Look, here are the people we saw.' They do it for entertainment."

Komor-a-kora: "We said to each other, 'Are we here just for their amusement?' "

"They conclude that white people are thieves. The relationship is similar to prostitution," says Turton. "The Mursi know they are looked down on. But to them the encounter is a commercial transaction. They are short of everything and cash is important."

Tourism has always been culturally destructive and exploitative. Hundreds of people once lived in hardship but security on St Kilda, 60 miles off the west coast of mainland Scotland, but the community collapsed after first missionaries and then tourist boats arrived in the 1920s. Within a few years of the first tourists, the community had disintegrated and those remaining on the island had to be evacuated.

Equally, the Himba in Namibia survived everything that a hostile arid environment could throw at them for centuries until they became a tourist attraction in the 1970s. Their communities were overrun and many Himba are now beggars and alcoholics.

These days, tribes are regularly diminished in the name of economic advancement. The refugee Burmese Kayan women in Thailand, who wear brass coils round their necks, each year attract thousands of tourists, who pay to visit them in their camps. Their communities are disintegrating as alcoholic dependency grows.

Governments also act inhumanely to encourage tourism. The Botswana government is putting out to tender for safari companies to build lodges with bore holes in the central Kalahari game reserve at the same time that the Bushmen - who have lived there for millennia - are forbidden to even use the existing ones. One safari lodge will have a water hole less than a mile from the Bushmen, who will be made to walk hundreds of miles to collect water.

The worst destruction of indigenous groups is often invisible, done by governments and the tourism industry exploiting tribal groups for their land. "Indigenous peoples are often removed from their ancestral lands to make way for tourist developments or to create national parks where animals take precedence over people," says Tricia Barnett, director of Tourism Concern.

The watchdog group is to publish a major report on the human rights abuses of tourism in September. "Tourism is violating left, right and centre all manner of the articles contained in the UN human rights declaration - land rights, dignity, respect, the right to privacy, cultural exploitation," says Barnett.

But above all, land everywhere is being claimed at the expense of indigenous people for the construction of hotels and golf courses, and for the creation of national parks and reserves.

"The onward march of tourism involves the arbitrary removal of people from their lands," says Barnett. "Tourists are becoming often unwitting collaborators in the exploitation of others. It is a competitive, resource-hungry industry, by its nature exploitative. International hotel chains and operators jostle to expand and out-price each other, and impoverished governments compete to attract business by offering cheap land and tax free investment. Indigenous groups are often the most vulnerable."

"Tourism is land hungry. It depends on unspoilt landscapes. Time and again the indigenous peoples have their land grabbed. They just don't come into the equation," says Rachel Noble of Tourism Concern.

But it is possible to get ethno-tourism right in an ethically sensitive way. Jonny Bealby, who runs Wild Frontiers, which has been taking small groups of people to stay days at a time in remote places such as the Hindu Kush in Pakistan, says many eco-tourism businesses are starting up.

"These are joint ventures with indigenous communities, like the Achuar [on the Peru/Ecuador border]. In the western Amazon, there are several eco-lodges where usually an agency sets up a partnership with a tribe. The company and the tribe each have a 50% stake. On the whole, they seem to be perfectly respectful of each other. The communities do it on their own terms. The ventures are on a manageable scale. It's fundamentally a meeting of equals. It comes down to scale and who is in control," says Bealby. "If [ethno-tourism] is done right it can benefit everyone."

Successful ventures, such as with the Akha hill tribe in Thailand, Aboriginal cultural tours in Australia, the Garifuna tourism group in Honduras and the Il Ngwesi Lodge in Kenya, which is 100% owned by local Maasai, are invariably grassroots-led and community-based.

"Tribal groups often feel that some tourists exploit them. It happens when they are being observed as if in a goldfish bowl. They do not like it when tourists stay in a swanky hotel and drive in and do not talk to them, then get in their Jeeps and go back," says Bealby

"That kind of thing happens a lot. But when it's small groups and the money goes direct to local people, then the benefits flow both ways."

• For more information on the Jarawa go to survival-international.org/jarawa.

In it together

Leading lights of ethno-tourism

Photograph: Alamy

Il Ngwesi Lodge Kenya

Perched on the edge of the Mukogodo escarpment, this is an award-winning, upmarket eco-ranch with timber floors flowing around tree trunks and an infinity pool. Guests can shower outside overlooking a waterhole, go on game drives, camel safaris and guided bush walks. The lodge is owned and run by the Il Ngwesi (which means "people of the wildlife") Maasai tribe of Laikipiak, who have lived on this land for centuries.

• ilngwesi.com.

El Descanso Costa Rica

El Descanso, in the Río Grande de Térraba river basin, is run by the Asodint indigenous organisation. Traditionally-designed cabins are set in tranquil surroundings and traditional food is on offer. Guests can visit ancient petroglyphs, the Catarata and Térraba rivers and other indigenous communities, learn about medicinal plants, play traditional games and buy local handicrafts. Profits are reinvested into the community.

• nacientespalmichal.com and actuarcostarica.com.

Photograph: Alamy

Garifuna Tourism Group Honduras

Located along the peaceful, undeveloped white sandy beaches on the north coast of Honduras on the Caribbean, the Garifuna communities offer grass roots tourism based on cultural exchange and interaction. Guests share in the vibrant local dance, food and music cultures, and learn about Garifuna's traditional fishing culture. The central tourism group ensures that communities are never over-saturated with tourists, so visitors feel like invited guests. The enterprise is owned by the Garifuna people.

• 00504 9277513 and +4480121, geo.ya.com/ENKEL.

Akha Hill Tribe, Chiang Rai Thailand

In mountainous northern Thailand, visitors stay in bamboo or mud bungalows overlooking a valley surrounded by tea plantations, rice fields, waterfalls and jungle. There is an open-air restaurant, a herbal sauna, and jungle treks with expert guides, including fishing, elephant rides, an overnight stay in a banana leaf house, and visits to other hill tribes. All profits go to the Akha Hill Tribe community and its education system.

• 0066 0899975505, akhahill.com.

Aboriginal Cultural Tours Australia

Aboriginal Cultural Tours take you to rarely seen areas of Adjahdura Land on the Yorke Peninsula of South Australia, walking with descendants of the original owners of the land, living with, talking with, and experiencing first-hand their rich culture. Guests visit Aboriginal communities, explore ancient cultural landscapes and mythological land formations and experience cultural ceremonies. Aboriginal Cultural Tours is owned and operated by indigenous people.

• diversetravel.com.au and aboriginalaustraliatravel.com.

These projects are all listed in Tourism Concern's Ethical Travel Guide, available to buy at tourismconcern.org.uk, 020-7133 3800.