Britain says a missing Hong Kong bookseller was likely abducted to mainland China in what it says is 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 a Hong Kong publishing company specializing in books critical of China's communist leadership who have gone missing in recent months only to turn up later in mainland China.

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.