Britain on Friday accused China of a serious breach of a longstanding bilateral treaty between the U.K. and China over the disappearance of Hong Kong bookseller Lee Bo. The U.K. government reportedly said that it believes Lee, a British citizen, was "involuntarily removed ... without any due process" under Hong Kong law.

British Foreign Secretary Philip Hammond said in a six-monthly report to Parliament on the state of freedoms in the former British colony, that Lee, who disappeared from Hong Kong in late December, was likely taken to mainland China against his will. Hammond reportedly said that current information indicates that Lee, who surfaced in China last month, was abducted. Lee and four of his colleagues — whose books were critical of the Chinese government and were banned in the mainland — disappeared late last year, sparking fears that Beijing was clamping down on Hong Kong's freedom of speech.

"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 said, according to Reuters, referring to the 1984 Sino-British Joint Declaration that paved the way for Hong Kong's 1997 return to China. As part of the treaty, China promised to safeguard Hong Kong’s freedoms.

"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," Hammond reportedly said.

China has not yet responded to Britain’s accusations, but had earlier said that Hong Kong's autonomy was fully respected and no foreign officials had the right to interfere, Reuters reported.

Last week, Chinese authorities indicated that three of the five Hong Kong booksellers were being investigated for unspecified "illegal activities."

In January, Lee’s wife told Hong Kong police that she had met him in China at a guesthouse. Lee looked healthy and in good spirits, she reportedly said, adding that he told her he was assisting in an investigation as a witness.

Also, last month Lee’s colleague, Gui Minhai, who was reported missing in October 2015 appeared on Chinese state TV where he confessed to his involvement in a 2003 fatal hit-and-run case. Gui, a Chinese national who also holds a Swedish passport, reportedly said that he had been on the run for 12 years after being convicted of drunk driving in August 2004.