HONG KONG, JUNE 25 -- The Chinese government today allowed the country's best-known dissident, Fang Lizhi, and his wife, Li Shuxian, to leave China after the couple spent more than a year sheltered in the U.S. Embassy in Beijing.

Chinese authorities said the two were allowed to leave after showing "signs of repentance" in a letter sent to the government and were going abroad to seek medical treatment. However, it was not clear if Fang and Li had in fact repented or what type of medical treatment was needed.

The surprise action removed one of the biggest obstacles to better U.S.-Chinese relations and was immediately praised by the White House as "a far-sighted, significant step" that would facilitate an improvement in ties between Washington and Beijing.

Diplomats said Beijing's decision appeared to be aimed at influencing the summit meeting of the world's seven major industrialized nations scheduled for July 9 to 11 in Houston.

The group is expected to consider easing economic sanctions imposed last year on China -- including suspension of World Bank loans totaling more than $500 million -- to protest the Beijing government's crackdown on an unprecedented democracy movement. Japan's foreign minister said Saturday that he would try to persuade other summit participants to ease the sanctions and called on Beijing to make a sign showing it would improve its human-rights record.

The move also came as Congress is set to debate an extension of China's most-favored-nation status, which allows Beijing low tariffs on exports to the United States. Many legislators, citing China's repression of dissent, oppose President Bush's decision to extend the status and want to block it in Congress.

Fang and Li flew out of China today en route to Britain on a U.S. Air Force C-135 transport plane from a Chinese military airbase south of Beijing. A British government spokesman said in London that Fang had been invited by the Royal Society, Britain's foremost scientific and academic body, to pursue his research. A White House spokesman said in Washington that Fang had accepted an invitation to teach at Cambridge University.

Fang, 54, an astrophysicist, and his wife, a physicist, had spent more than a year in the U.S. Embassy in Beijing, where they took refuge on June 5, 1989, after the Chinese army crushed the student-led democracy movement. Western diplomats said at the time that Fang's life was in danger.

Although Fang denied direct involvement in the democracy movement, Beijing accused him and Li of instigating unrest and charged them with "counterrevolutionary activities" and attempting to overthrow the government. After Fang and Li fled to the embassy, authorities said the two scientists could not leave the country without admitting guilt and promising not to engage in activities directed against the government.

In announcing the release today, the Ministry of Public Security said that "in view of the signs of repentance" by Fang and Li and in view of "their illness and out of humanitarian considerations," the government had "decided to allow them to go abroad for medical treatment in line with China's policy of leniency towards those who participated in the disturbances."

The ministry said Fang and his wife had written to the Chinese authorities admitting that they had violated the country's constitution by opposing China's "four cardinal principles," which include requirements that all Chinese support the Communist leadership and the ideas of the late party chairman Mao Zedong. The ministry said the two had also agreed "not to engage in activities directed against China."

It could not be confirmed, however, whether Fang and Li had made such promises, and it was unclear what kind of medical treatment Fang or his wife were seeking. None of today's official statements said either was ill, and their elder son, Fang Ke, a student at Detroit's Wayne State University, said told CBS-News that he knew of no medical problems his parents might have.

The release was welcomed in Washington, where White House spokesman Marlin Fitzwater issued a statement praising it for what was described as "reasons of Dr. Fang's health and well-being and to permit Dr. Fang to pursue his important research in astrophysics." The statement added: "This humanitarian action is a far-sighted, significant step that will improve the atmosphere for progress in our bilateral relations."

Ties between Washington and Beijing have been strained since the Chinese army killed hundreds of civilians in its crackdown on democracy demonstrators around Beijing's Tiananmen Square and Bush, in protest, withdrew U.S. approval of World Bank loans, suspended military sales to China and barred high-level diplomatic contacts. Bush, however, tried to maintain a dialogue with Beijing, twice sending his national security adviser on secret trips to China. Earlier this year, his administration conceded it was disappointed with China's refusal to ease repression.

China has made a number of recent gestures apparently designed to relax strictures on civil rights, but critics have said the measures are largely cosmetic. Last January, authorities lifted martial law in Beijing -- but police controls were retained across the city. In May, martial law in the Tibetan capital of Lhasa was lifted, but repression continued.

More than 800 political prisoners have been released in recent months, but thousands more are believed still held in prisons, detention centers and labor reform camps. A number of well-known dissidents remain in prison, including student leader Wang Dan and political activist Ren Wanding.

Fang gained political prominence in late 1986 when he served as vice chancellor of the prestigious China Science and Technology University at Hefei in eastern China, where students began the first demonstrations for democracy, which later spread across the country. He was accused of helping incite the protest and in early 1987 was expelled from the Communist Party.

Senior Chinese leader Deng Xiaoping personally criticized Fang at the time in a secret meeting of party leaders and said he could find no evidence of Marxist thinking in Fang's writings and speeches. Chinese leaders appeared to be particularly offended when Fang alleged that top leaders and their children benefited from corruption.

Fang was one of only a few dissidents who dared to speak openly in favor of democratic reforms even before student demonstrators raised the issue on a large scale. In February 1989, Bush, during a visit to Beijing, invited Fang to a banquet attended by Chinese leaders, but police blocked his entry.

During their subsequent long stay at the embassy, U.S. officials refused to answer any questions about Fang and Li, referring to them only as "the guests."