A young Tibetan writer secretly sentenced to one of the harshest jail terms in recent years in the Himalayan region over a book he never published has appealed to the United Nations for help.

In a letter smuggled out from the Chusul prison on the outskirts of the Tibetan capital, Lhasa, and seen by The Times, Dolma Gyab said his aim was to draw attention to the situation in Tibet and to seek help against a 10-year jail term that he said was unfair.

News of his incarceration coincides with a renewed crackdown on the Internet by Chinese authorities, including the closure of one of the most popular online forums among intellectuals and students.

In what is believed to be the first letter ever smuggled out of a Tibetan jail, the 29-year-old teacher wrote: "I have written a book which was not yet published. In this book I wrote about democracry, freedom and the situation of Tibet. That is the main reason for my conviction, but according to Chinese law this would be not enough reason to give me such a sentence."

His case has been kept secret ever since his arrest in March 2005 in Lhasa, where he was teaching history at a city middle school.

He had written a 57-chapter book that he had called "Restless Himalayas" and had also begun a book on Tibetan geography and which was believed to touch on such sensitive topics as the locations of Chinese military camps in Tibet. These unpublished papers were apparently found in his home.

His letter to the United Nations Committee for Human Rights says these papers were the main reason behind his conviction. The Lhasa People’s Intermediate Court on 16 September last year sentenced him to 10 years in prison on charges of endangering state security.

The former history teacher wrote: "They can kill me, but they cannot kill the love of nature, science and geography. I want to keep up my courage… I would like to draw attention to this situation and ask you to help me."

Since the Dalai Lama fled into exile in 1959 amid a failed anti-Chinese uprising, the authorities are believed to have jailed hundreds, if not thousands, of Tibetans for their resistance to rule from Beijing.

Tibet experts said this was the most severe sentence to be handed down for several years. One Tibet expert expressed astonishment that the case could have been kept secret by the authorities for so long.

When the UN Special Rapporteur for Torture, Manfred Nowak, made his unprecedented two-week visit to China last November, he was given access to prisoners he asked to meet at Chusul prison. But the case of the young writer was not then known. In his letter, Mr Gyab writes that during the visit by the UN official he was removed and hidden from the visitors. "Therefore I did not have a chance to talk about the real situation here and my unfair trial."

Mr Gyab was born in 1976 in Arik county of the ethnically Tibetan province of Qinghai where most people are nomad farmers.

He studied history and geography at Qinghai Normal University and graduated in 1999. He followed postgraduate studies at prestigious Beijing University until 2003 when he went to India and studied English in Dharamsala, the town that has been home to the Dalai Lama and his followers in exile since he left China.

Returning to Tibet in 2004, the young man took up a job teaching history at a middle school in Lhasa.

Stepping a crackdown on the independent discussion on the Internet, officials late last week closed down blogs maintained by Tibetan writer Woeser whose works are already banned in China.

Her blogs at http://oser.tibetcul.net/ and http://blog.daqi.com/weise/ that had displayed her essays and poetry along with work by her husband, renowned author Wang Lixiong, addressed such sensitive issues as the recently completed railway to Lhasa and the 40th anniversary of the Cultural Revolution when most Tibetan temples were destroyed by rampaging ultra-leftist Red Guards.

The crackdown came after she had posted a picture of the Dalai Lama on her blog to mark his birthday on July 6. She told The Times: "This had to happen sooner or later. I will not start another blog but I will continue with my own writing."

More than 100 leading writers and dissidents yesterday issued a letter to decry the closing of the Century China website that had been one of the few refuges for relatively unfettered views in a country where censorship stretches into every corner.

Over the past six years, Century China was popular forum for liberal critics of the Communist Party, relaying discussions for political and social ills and calls for political relaxation.

The petition said: "The shutdown of Century China is just another instance of the Chinese government suppressing the freedom of its people." It called the website "the one spirital home we had in the cyber world".