British Prime Minister Margaret Thatcher refused today to acknowledge China's claim of sovereignty over the capitalist colony of Hong Kong.

Thatcher did tell a news conference that her three-day visit here resulted in an agreement with Communist leaders to open negotiations on Hong Kong's future "with the common aim of maintaining the stability and prosperity" of what now is a British colony.

The issue of sovereignty over territory that China lost to Britain in a series of 19th century military defeats is expected to be the initial focus of negotiations.

Premier Zhao Ziyang publicly staked China's claim to the wealthy commercial center yesterday, saying Peking "must recover sovereignty," presumably after the 99-year British lease to part of the territory expires in 1997.

"The Chinese government's position on the recovery of the sovereignty of the whole region of Hong Kong is unequivocal and known to all," declared the official New China News Agency today.

Thatcher, however, declined to endorse this unilateral claim at her news conference, which had been expected to be a forum for reassuring jittery Hong Kong investors as well as the largely ethnic-Chinese population of 5.2 million.

She said the issue of sovereignty is guided for the time being by three treaties, which awarded the territory in parcels -- 90 percent on a lease arrangement lasting another 15 years and the rest ceded in perpetuity.

"There are, in fact, treaties in existence," said the prime minister. "We stick by our treaties unless we agree on something else. At the moment, we stick by our treaties."

It is unclear if Thatcher was expressing merely a bargaining position or hard political principle. London is thought to be ready to give up dominion over Hong Kong but wants legal guarantees to administer the banking and trade center.

Communist officials disavow the three Hong Kong treaties, saying they were "unequal" because they were signed by a corrupt, tottering dynasty in a different era.

Nevertheless, Peking has a great interest in Hong Kong's prosperity -- it provides China 40 percent of its annual foreign exchange earnings -- and Communist leaders have indicated a willingness to maintain the current capitalist system, but under a Chinese flag.

Thatcher, who met with Premier Zhao and leader Deng Xiaoping, seemed encouraged by their approach. Asked what assurances she had for Hong Kong's future, Thatcher repeatedly stressed the importance of Peking's commitment to the political and financial well-being of the region.

"I think the people of Hong Kong are very realistic," she said. "I think they'll look at the joint statement and they'll note that the common aim is to maintain the stability and prosperity of Hong Kong."

London has long pressed for formal negotiations on Hong Kong's fate to remove the uncertainty that inhibits long-term investments. Since Peking began leaking plans to reclaim the territory, the once booming economy has seen stock prices plummet, land values fall and the currency's exchange rate hit a historic low.

Thatcher said negotiations will be speeded in the hope of allaying concerns of residents and businessmen.