The Dalai Lama claimed yesterday that Beijing was planning the mass settlement of 1 million ethnic Chinese people in Tibet after the Olympics with the aim of diluting Tibetan culture and identity.

Tibet's exiled spiritual leader also claimed that some of Asia's most important rivers which flow from the Tibetan plateau are being polluted and diminished by careless industrialisation and unplanned irrigation.

The Dalai Lama made the claims in an interview with the Guardian after a meeting yesterday with Gordon Brown at Lambeth Palace. He said the talks had been detailed and the prime minister had been helpful "in spite of his difficulties". The Dalai Lama said: "He met me and he showed genuine concern and he wants to help."

Downing Street said the discussion focused on talks due next month on Tibet's future between Tibetan representatives and Beijing officials. The prime minister is said to have stressed the importance of the Dalai Lama's pledge to oppose violence, not seek Tibetan independence, nor support a boycott of the Beijing Olympics.

The Dalai Lama said he feared the Chinese authorities could take a tougher line on Tibet after the Olympics, and possibly flood it with Han Chinese, the world's largest ethnic group.

The Dalai Lama said he had been informed by Tibetan residents that large areas of empty land had been marked out, as if for construction, in the past two years. "Then last year we received information - after the Olympics 1 million Chinese are going to settle in the autonomous region of Tibet," he said, adding the information came from a "military source" in Tibet.

"There is every danger Tibet becomes a truly Han Chinese land and Tibetans become an insignificant minority. Then the very basis of the idea of autonomy becomes meaningless."

There has been an increasing influx of Chinese settlers into Tibet in recent years as transport has improved, but the exact figures are a matter of dispute. According to an official census in 2000, there were 2.4 million Tibetans in the region and 159,000 Han Chinese. The government in exile says there are many more Chinese if migrant workers and soldiers are counted. The Dalai Lama has said there is a Han majority in Lhasa, the regional capital.

China has denied carrying out any deliberate settlement policy aimed at the dilution of Tibetan culture and points instead to the benefits brought to the region by economic development and investment.

The Dalai Lama claimed over-settlement and over-exploitation of Tibet was threatening the quality and flow of rivers flowing out of the Tibetan highlands, including the Yangtze, the Yellow River, the Indus, the Mekong and the Ganges.

"Due to carelessness these waters have been polluted and also reduced, and I think billions of people's lives depend on these rivers," the Dalai Lama said. "[There has been] mining without proper care, deforestation ... irrigation without proper planning. In some valleys, new diseases have developed which some specialists believe is the result of water pollution."

Lhasa is now relatively quiet since protests were put down by Chinese troops in March, and the Dalai Lama has threatened to resign if the unrest turns to violence. But he said the Tibetan commitment to non-violence might not outlast him.

"Now there are signs of frustration among Tibetans, not only young monks," the 72-year-old Buddhist leader said. He said Tibetans were now telling themselves: "While the Dalai Lama remains, we have to follow his advice. That means non-violence. After him, we ourselves will take appropriate action."

The next talks between representatives of the Dalai Lama and China are scheduled for Beijing on June 11. On Wednesday, envoys of the Tibetan leader visited the Chinese embassy in London to offer his condolences for the dead from this month's earthquake in Sichuan.

Asked what he thought Gordon Brown should tell the Chinese president, Hu Jintao, when he attends the Olympic closing ceremony in August, the Dalai Lama said: "If within two months it gets more positive then the prime minister must give encouragement and appreciation. If things get worse, the prime minister will have to speak out."