Hong Kong(CNN) Britain says that a missing man associated with a controversial Hong Kong publishing house was taken to mainland China against his will.
In a half-yearly report to the UK Parliament on Hong Kong, Foreign Secretary Philip Hammond called for the immediate return of Lee Bo, a British citizen, and said his case was a "serious breach" of a bilateral treaty between the United Kingdom and China.
"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," the foreword to the report said.
Lee disappeared from Hong Kong in December, sparking mass protests amid allegations Chinese police, who are not permitted to act in the autonomous city, detained and spirited him across the border.
Responding to questions about Lee's case, the Hong Kong government said in a statement, "Police have been seeking assistance from the mainland authorities and have received replies which the police have made public. Any suggestion that 'Mr. Lee was involuntarily removed to the mainland' remains speculative."
China's Foreign Ministry accused Britain of making "groundless accusations" about its former colony.
"The so-called "responsibility" that the British side claimed to have over Hong Kong does not exist. We ask the British side to mind its words and actions and stop interfering in Hong Kong affairs," said Hong Lei, a spokesman for China's Foreign Ministry.
No escape? China ignores borders in its pursuit of critics
Since his disappearance, letters purportedly written by Lee have been published in Hong Kong newspapers, and his wife, Choi Ka-ping, said she has met with him in a guesthouse on the mainland.
Hong Kong police said in January they were told Lee was in mainland China and requested to meet with him.
Under the "one country, two systems" policy agreed as part of Britain's 1997 handover of its former colony to China, the 7 million residents of Hong Kong -- defined as a "Special Administrative Region" of China -- have greater, guaranteed civil liberties than those in the mainland.
But concerns have grown that some freedoms are being eroded as Beijing asserts more control.
Lee was a shareholder in a bookstore owned by publisher Mighty Current, best known for gossipy titles about China's ruling elite.
Four others linked to or employed by the publisher also went missing, including owner Gui Minhai, a Swedish passport holder, and they have since been confirmed to be in police detention in China.
"Nearly four months after the men first vanished, we still don't know where they are detained, on what charges they are being investigated, and whether they are allowed to see their lawyers or families," said William Nee, China researcher at Amnesty International.
"The Chinese authorities seem to think that if they can get detained people under their control to write implausible letters or call family members saying that they are proactively 'cooperating with investigations' they can do away with due process and human rights."