The 29,035-foot peak is battered by harsh weather and wreathed in thin oxygen, presenting a physical and technical challenge to the torch crews. The mountain also straddles the political border between Chinese-controlled Tibet and Nepal, home to Tibetan exiles and activists.

Activist groups have criticized the Everest run as an attempt by Beijing to lend legitimacy to Chinese rule. "Beijing is using the Olympics torch ceremony, which should stand for human freedoms and dignity, to bolster its territorial claim over Tibet," John Ackerly, president of the Washington-based International Campaign for Tibet, said in a statement.

Meanwhile, the Chinese government has said that the Everest relay would be a show of international sportsmanship, as well as a point of national pride.

"The torch relay to Mount Everest is a highlight of the whole relay, and it also represents the idea of green Olympics, high-tech Olympics and people's Olympics," Beijing Vice Mayor Liu Jingming told reporters in Beijing. He promised a successful ascent even if the weather was bad, saying a test-run went well last year.

Beijing has been secretive about the Everest leg of the symbol-freighted, popular Olympic relay. The Everest run is a side spur of the main event, a second torch that will be carried up the mountain while the relay is in Southeast Asia or elsewhere in China.

Beijing Olympic organizers have not released an exact date of the ascent, but preparations point to late April or early May.

The north side of Everest, which is in Chinese territory, contains some of the most difficult routes to the summit. The south side, through Nepal, is the most popular way to the top. China has tried to pressure Nepal to close the south side as well, according to Web site everest.net.

In September 2006, a Tibetan man and Buddhist nun were shot and killed by Chinese border guards at the 19,028-foot Nangpa La Pass, a common escape route for Tibetans fleeing to Nepal. The shootings, which China said were in self-defense, were caught on video by climbers at a base camp for the nearby Cho Oyu mountain.

The letter sent to expedition companies this week by the government's China Tibet Mountaineering Association, which issues permits for Everest, said climbs of that mountain and Cho Oyu should be postponed until after May 10.

The letter, which was posted on a foreign mountaineering Web site MountEverest.net and verified by the association, cites "heavy climbing activities" as among the reasons but does not mention the torch.

Zhang Mingxing, general secretary of the association, said his group would still welcome several hundred climbers but suggested that most would be in the August to October climbing season. Besides, he said, "the climate in Tibet this year is a bit unusual. It is still snowing here and the wind is pretty big, so it is better to postpone the climbing."

Mountaineering groups, incensed by the decision, said they had been told that the relay was the main reason for the postponements.

"No applications for climbing Mount Everest between March and June are being accepted" because of the torch relay, said Li Hua with the Tibet Polar Land Exploration Tourist Co. in the Tibetan capital, Lhasa. "No matter whether you're an individual or group, it's impossible to get permission to climb the mountain during the period."