The UK government says a missing Hong Kong bookseller was probably abducted to mainland China, accusing Beijing of a "serious breach" of the treaty under which it took control of the city.
Philip Hammond, the British foreign secretary, said in a report on Hong Kong affairs released on Thursday that Lee Bo, a UK citizen, was "involuntarily removed" to the mainland.
Lee is one of five men linked to the Hong Kong publishing company Mighty Current Media and its Causeway Bay Bookshop who have gone missing in recent months, only to turn up later in mainland China.
Their disappearances have raised international concern. Lee is chief editor of Mighty Current, which specialises in books critical of China's communist leadership that are banned in the mainland but popular with visiting Chinese tourists.
"The full facts of the case remain unclear, but our current information indicates that Mr Lee was involuntarily removed to the mainland without any due process under Hong Kong SAR law," the report said.
"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," the report added.
Hammond said while visiting Beijing in January that he made urgent inquiries with Chinese authorities about Lee's whereabouts.
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Lee's disappearance at the end of December sparked international concern because he was last seen at his company's Hong Kong warehouse and did not have his mainland China travel permit with him, raising suspicions he was snatched by Chinese security agents who crossed over from the mainland.
He later sent letters to his wife saying he was helping with an investigation on the mainland.
The Joint Declaration is the treaty signed in 1984 between Britain and China safeguarding Hong Kong's rights and freedoms after Beijing took power in 1997.
Under the "one country, two systems" principle, Hong Kong retains a high degree of control over its own affairs, including law enforcement.
Lee's colleague Gui Minhai, a Swedish citizen, disappeared from Thailand in October and turned up last month on Chinese state TV to confess to a decade-old fatal drunk driving accident.
Hong Kong police said last week the three other men are being held on the mainland for an investigation into unspecified "illegal activities".