Britain says a missing Hong Kong bookseller was likely abducted to mainland China in what it called a "serious breach" of the treaty under which Beijing took control of the city.

British Foreign Secretary Philip Hammond said in a twice-yearly report on Hong Kong affairs released Thursday that Lee Bo was "involuntarily removed" to the mainland.

Lee, a British citizen, is one of five men linked to Hong Kong publishing company Mighty Current Media and its Causeway Bay Bookshop who have disappeared in recent months only to turn up later in mainland China.

Lee is chief editor of Mighty Current, which specialized in books critical of China's communist leadership that were banned in the mainland but popular with visiting Chinese tourists.

It's Britain's strongest statement so far on the case.

Hammond said while visiting Beijing last month that he made urgent inquiries with Chinese authorities about Lee's whereabouts.

Lee's disappearance at the end of December sparked international concern because he was last seen at his company's Hong Kong warehouse and didn't 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, though some believe he was coerced.

"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.

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 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."