Britain said on Friday that one of the five missing Hong Kong booksellers, Lee Bo, had likely been "involuntarily removed" to China from Hong Kong, constituting a "serious breach" on the agreement signed with Beijing in 1984 before the city was handed back to China in 1997.
Lee, 65-year-old dual citizen of Britain and China and owner of a publisher and bookstore specialising in books critical to China’s Communist Party leaders, disappeared from Hong Kong on December 30.
"Our current information indicates that Mr Lee was involuntarily removed to the mainland without any due process under Hong Kong SAR law," British Foreign Secretary Philip Hammond wrote in the foreword of a six-monthly report to parliament on the state of freedoms in the former British colony.
"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 added, referring to the 1984 treaty.
It was the strongest comment of the UK on the issue so far.
China has previously said Hong Kong's autonomy was fully respected and no foreign officials had the right to interfere.
The three other booksellers, Lui Por, Cheung Chi Ping and Lam Wing Kee, who went missing on October, “were suspected to be involved in a case relating to a person surnamed Gui, and were involved in con the Mainland,” a letter Chinese police sent to the Hong Kong police said this month.
Gui Minhai, the fifth missing bookseller with a Swedish passport, disappeared last October in Thailand.
On January, he appeared on Chinese state television, saying he had surrendered to the authorities due to a fatal drunk driving charge from more than a decade ago.
Senior diplomats say that China's reluctance to provide information on the issue and its refusal to allow British and Swedish envoys access to Lee and Gui is fuelling a diplomatic crisis, Reuters reported.
"The unexplained disappearance of five individuals associated with a Hong Kong bookstore and publishing house has raised questions in Hong Kong," Hammond said.
"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 wrote.
There are around 3.7 million British citizens in Hong Kong, a city of 7.2 million.