Steve Marsh and a British police officer who asked not to be identified were having breakfast at Advance Base Camp beneath Mount Chu Oyu, near Everest. “As we watched, first I heard a shot, then I saw one [guard] stop and there was a second shot and a third,” Mr Marsh told The Times.

“The rearmost of the group fell to the ground. Someone helped them up and they continued for another 30 metres. Then there was another shot and someone fell and they left that person behind in the snow. Through the telescope we could clearly see it was a body.”

The International Campaign for Tibet (ICT) said that the victim was a nun in her mid-twenties from central Tibet who was among 73 refugees trying to flee via Nepal to India where the Dalai Lama, the exiled Tibetan leader, has been based since 1959.

Mr Marsh, 44, could not confirm the victim’s identity because he and his colleague were watching with about 100 other foreign mountaineers from 300 metres away. But, he said, they clearly saw the guards shooting at a group of about 25 Tibetans heading towards the 18,300ft Nangpa La pass — a common route for traders, yak herders and refugees.

The incident is an embarrassment for China, which says it has brought peace and prosperity to Tibet since its troops entered the region in 1950. Chinese guards often detain some of the 3,000 Tibetans who try to cross into Nepal every year, but this was the first time guards had shot dead anyone in front of such a large number of foreigners.

“We were shocked,” said the police officer, who wants to return to Tibet. “You expect to encounter death on the mountain. But you don’t expect to see someone shot in the back when they’re posing no threat to anybody.”

The guards ran past the body as they pursued the group then returned to inspect it but did not remove it until 30 hours later, he said.

Within an hour of the shooting about 30 Chinese guards entered the Advance Base Camp and shortly afterwards led 15 Tibetan children through it, he said. “The kids looked scared, but other than that healthy,” he said. There was no immediate comment from the Chinese Government. A third climber said that the Chinese Embassy in Kathmandu had contacted his group for information.

The ICT said that 43 of the refugees had made it over the border, but the whereabouts of the other 30 was not clear. The United Nations High Commissioner for Refugees said it had sent a representative to escort the 43 to Kathmandu. Under an informal agreement with Nepal, Tibetan refugees are granted safe passage through the country to India. However, Nepalese police have handed some refugees back to China.

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