Image copyright AFPThe UK has called the apparent abduction of a Hong Kong bookseller a "serious breach" of the treaty on the city's return to China.
Foreign Secretary Philip Hammond said Lee Bo, a British citizen, was "involuntarily removed... without any due process" under Hong Kong law.
Lee is among five booksellers who have disappeared in recent months.
In the 1984 Sino-British Joint Declaration, China promised to safeguard Hong Kong's freedoms.
Read more: Hong Kong's missing booksellers and 'banned' Xi Jinping book
Mr Hammond made his comments in the latest report from the British Foreign Office on Hong Kong affairs, which the foreign secretary files to parliament every six months.
Mr Lee, who disappeared in December, is a shareholder in Mighty Current, a publishing house specialising in books critical of senior Chinese leaders, and is linked to the Causeway Bay Books shop which sells the titles.
The disappearance of the booksellers have been a cause of major concern in Hong Kong where many believe they were taken to the mainland to face punishment.
Mr Lee later allegedly sent a letter to his family saying he had gone to the mainland voluntarily to attend to certain matters, but did not elaborate.
Another missing bookseller, Gui Minhai, appeared on Chinese state television saying he had given himself up to authorities for an old drink-driving conviction.
Mighty Current publishing house disappearances
1. Lui Bo, General Manager, goes missing in Shenzhen, 15 October
2. Cheung Jiping, business manager, 32, goes missing in Dongguan, 15 October
3. Gui Minhai, co-owner, 51, goes missing in Thailand, 17 October
4. Lam Wingkei, manager, 60, last seen in Hong Kong, 23 October
5. Lee Bo, shareholder, 65, goes missing in Hong Kong, 30 December
http://www.bbc.co.uk/news/world-asia-china-35557226#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
