It is one of the most romantic legends in mountaineering - the story of how a young Sherpa named Tenzing Norgay tended his father's yak herds on a high mountain pass below Everest before becoming, with Sir Edmund Hillary, the first man to reach its summit in 1953.

But while Hillary and the expedition's leader Lord Hunt both believed the Sherpa had been born in a remote mountain village in Nepal, a new book by American mountaineer Ed Webster claims that not only was Tenzing born in Tibet, but he spent much of his childhood there. The world's most famous Sherpa was not really a Sherpa at all.

Even after Tenzing's death in 1986, the truth was considered too sensitive to disclose, not least for fear of embarrassing the Indian government which had supported Tenzing after his ascent. It would have handed a propaganda coup to the Chinese authorities in the Tibetan capital Lhasa that a 'Chinese climber' was the first to climb Everest. But now Webster has been given permission by the family to reveal the truth about Tenzing's real origins.

Throughout his life, Tenzing remained vague about his background. In his autobiography, Tiger of the Snows, he obscured the truth of his childhood without quite denying it, telling ghostwriter James Ramsey Ullman he grew up in the village of Thame, in Nepal. In fact, his parents migrated there during the early 1920s after a period of financial hardship and debt to a local Tibetan governor.

Tenzing, however, was more forthcoming about his birthplace, saying: 'I was born in a place called Tsa-chu, near the great mountain of Makalu, and only a day's march from Everest.' Tenzing also explains that when he was born, his mother had been on a pilgrimage to the nearby monastery at Ghang La, the name of Tenzing's house in Darjeeling.

When Tenzing climbed Everest in 1953 he was hailed by the Nepali government in Kathmandu as a local hero who happened to live in India. Nepal's fledgling constitutional monarchy feared political domination by the new Indian republic and both countries saw great propaganda value in claiming Tenzing, the first humble-born Asian of the modern era to achieve global fame, as their own.

Tenzing's caution about revealing his true origins was partly explained by this political wrangling. 'After we climbed Everest,' Hillary said, 'and Tenzing was invited to England, we were really in a jam because Tenzing had no passport.' The crisis was averted only when the Indian Prime Minister Pandit Nehru stepped in and personally ensured Tenzing received an Indian passport - something for which the Nepalese authorities never forgave him. Nehru became Tenzing's patron and authorised the establishment of a mountaineering school in Darjeeling, which Tenzing helped to run. To avoid political embarrassment, Tenzing described himself as 'born in the womb of Nepal and raised in the lap of India,' but that was far from the whole story.

Now the full story has been revealed in Webster's Snow in the Kingdom, which describes an expedition in 1988 to the rarely visited East Face of Everest which approaches the mountain through the Tibetan Kharta Valley where Tenzing's home village of Moyun is located.

Included in the team was Tenzing's oldest surviving son, Norbu, who was born in the Sherpa community at Darjeeling in India where his father had started his career as a climber after migrating there in the early 1930s.

Norbu, like most Sherpas, knew all about his father's secret and, while in Tibet, he was able to meet long-lost relatives, including Tenzing's half-brother Tashi, and also to solve the riddle of Tenzing's birthplace.

'If Tenzing had come out with the truth that he was, in fact, a Tibetan, he would only have magnified his nationality problems, greatly disappointing India where he then lived,' Webster told The Observer. 'It's possible some Sherpas might have ridiculed him as something of an imposter, and as a social and cultural inferior.' All of Tenzing's three wives were Sherpas and he remains a potent hero in Darjeeling and the Khumbu.

'I believe Tenzing was a sensitive and a sincere man,' says Webster. 'His writings make this clear, so Tenzing never lied outright about his family origins - but he never told the full truth either. Perhaps he believed he was simply a mountaineer, and nationality didn't matter.'

During the expedition in 1988, Webster identified the monastery at Ghang La as Namdag Lhe Phodang - the 'pure god's palace' - high in the Kama Valley of Tibet. The region is very sacred to Tibetan Buddhists, regarded as a 'heavenly refuge' in times of war and famine. Tsa-chu, or more accurately Tshechu, is another holy site in a remote side-valley not far from Ghang La. The yak pastures around the monastery command a superb view of Everest and are almost certainly where Tenzing spent his childhood summers. Both the family house and the monastery were destroyed following the Chinese invasion in 1950.

Ironically, the Sherpa people originated in Kham in eastern Tibet, more than 1,000 miles from Everest, and migrated to Nepal in the sixteenth century. They still number only a few thousand in a Nepali population of 22 million but are world famous for their contribution to mountaineering.

While the Sherpa homeland is considered to be the Solu-Khumbu region of Nepal south of Everest, the Sherpas have always had strong cultural and economic links with Tibetans across the border. Tenzing's cousin, a famous reincarnate lama called Ngwang Tenzin Norbu, founded monasteries at Rongbuk on the Tibetan side, and at Tengpoche on the Nepali side. Tenzing Norgay's cousin also gave him his name, which means 'fortunate supporter of religion'.

The tradition of mountaineers hiring Sherpas began in Darjeeling at the start of the twentieth century; the hill men quickly proved the most reliable and physically capable porters. The best Sherpas were termed 'Tigers' and awarded medals. Mountain tourism proved lucrative and more migrated from the Everest region to India. Until 1949, Nepal remained closed to almost all Europeans.

For Tenzing, whose parents were struggling to make a new life for themselves in the Khumbu, Darjeeling offered a chance for economic success, but his early years there were plagued by money problems as he sought to make his mark as a porter for Western mountaineers. Tenzing never saw his father again, and didn't return to the Khumbu to see his mother until 1952. When news of his sudden fame reached his homeland in Tibet, he was overwhelmed by 'all sorts of relatives I had never seen or heard of before'.

Now, Sherpas earn thousands of dollars a year helping Western clients on Everest but, with the opening of Nepal to tourism, they no longer migrate to Darjeeling to look for work. Many Sherpas have become wealthy in a country where per capita income is $200 a year. The influx of tourists to the region Tenzing helped make famous has caused environmental problems, but has also helped alleviate the economic hardship caused by the Chinese occupation of Tibet.

After the invasion of Tibet in 1950, China also claimed sovereignty of the Khumbu region of Nepal, which it saw as ethnically Tibetan.They mounted a number of Sino-Tibetan expeditions to Everest following the occupation for their propaganda value and still control access to the mountain. Kate Saunders of the Tibet Information Network told The Observer: 'The Chinese will understand very well the propaganda value of Tenzing's birth. They have never wasted an opportunity to stress Chinese sovereignty in Tibet.'

Perhaps Webster's most intriguing claim is that the seven-year-old Tenzing Norgay may have met George Mallory, who disappeared on Everest in 1924 and whose body was discovered last year. According to the diary of fellow climber Guy Bullock, during the first expedition to Everest in August 1921, Mallory spent a day at Tenzing's home village before trekking through the summer pastures used by Tenzing's family to the foot of the mountain's East Face.



Tall tales from the world's highest peak

First identified as the world's highest peak in 1856 by the Survey of India and dubbed 'Peak XV'.



The name Everest was suggested by Colonel Andrew Waugh, Surveyor General, to honour his predecessor, Sir George Everest.



Everest actually pronounced his name Eve-rest. So we've been saying it wrong for 135 years.



Tibetans and Sherpas call the mountain Chomolungma, usually translated as 'Mother Goddess of the Earth', although Tenzing Norgay's mum said it meant the 'Mountain So High No Bird Can Fly Over It'.



The true height of Everest is still controversial. Scientists in America announced in November 1999 that the true height was 8,850m, two metres higher than the previous agreed standard 8,848m.



First climbed via the South Col on 29 May 1953 by Edmund Hillary and Tenzing Norgay. Since then there have been more than 1,300 ascents and over 170 deaths.



Last spring 130 people reached the summit, 80 of them via the South Col.



Sherpa Babu Chiri reached the summit in 16 hours 56 minutes from Base Camp.



Overcrowding on the mountain has led to punch-ups over campsites and the use of fixed ropes that now litter it.



Weirdest thing on the summit: the umbilical cord of French mountaineer Thierry Renault's son.



Weirdest act on the summit: an American climber performed rodeo rope tricks wearing a cowboy outfit.