Britain said on Friday that a missing Hong Kong seller of gossipy books on China's leaders had likely been "involuntarily removed" to China from Hong Kong, constituting a "serious breach" of a longstanding bilateral treaty between the U.K. and China.

China's Foreign Ministry gave no immediate response to a faxed request from Reuters for comment on the British report.

In a six-monthly report to parliament on the state of freedoms in the former British colony, British Foreign Secretary Philip Hammond wrote that Lee Bo, a British passport holder who disappeared from Hong Kong in late December, was likely taken to China against his will.

"Our current information indicates that Mr Lee was involuntarily removed to the mainland without any due process under Hong Kong SAR law," Hammond wrote in a foreword.

It was the strongest indication so far by London, that Lee, who surfaced in China last month, was abducted, though Hammond didn't specify by whom, how, or give any further details.
 "This constitutes a serious breach of the Sino-British Joint Declaration on Hong Kong and undermines the principle of "One Country, Two Systems" which assures Hong Kong residents of the protection of the Hong Kong legal system," Hammond added, referring to the 1984 treaty that paved the way for Hong Kong's 1997 return to China.

China has previously said Hong Kong's autonomy was fully respected and no foreign officials had the right to interfere.

Besides Lee, four of his bookselling associates have also gone missing over the past few months including Gui Minhai, a Swedish national who disappeared from the Thai seaside resort town of Pattaya late last year and who last month made a tearful confession on Chinese state television to a fatal drink-driving incident over a decade ago.

Chinese authorities indicated last week that three of the five Hong Kong booksellers who went missing were being investigated for unspecified "illegal activities"

China's reluctance to provide information and its refusal to allow British and Swedish envoys access to Lee and Gui - a breach of international conventions - is fuelling a diplomatic crisis, several senior diplomats told Reuters.

"The unexplained disappearance of five individuals associated with a Hong Kong bookstore and publishing house has raised questions in Hong Kong," Hammond said.

The case has raised concerns among Hong Kong's large number of ethnic Chinese who carry foreign passports, and the apparent inability of foreign governments to get access to them should they get into trouble with China. There are now around 3.7 million British passport holders in the city of 7.2 million.

"We urge the authorities in Hong Kong and Beijing to take the necessary steps to maintain confidence in the system and the sanctity of the rights, freedoms and values it upholds," wrote Hammond.

(Additional reporting by Ben Blanchard in Beijing; Editing by Michael Perry)