BEIJING (REUTERS) - China expressed anger on Thursday (Oct 13) after Britain's Foreign Secretary said he continued to have concerns about legal interference by Beijing in Hong Kong despite pledges to the contrary.

British Foreign Secretary Boris Johnson said in a report on its former colony that he had specific concerns about the "integrity of Hong Kong's law enforcement", which is separate from mainland China in the "one country, two systems" arrangement under which Hong Kong returned to China in 1997.

Mr Johnson said the case of Mr Lee Bo and four other Hong Kong booksellers who went missing and were subsequently found to have been detained by China was a serious breach of the Sino-British joint declaration on Hong Kong that undermined "one country, two systems".

"Although Lee Po has now returned to Hong Kong, the issues raised by the case remain of concern," he said, using an alternate spelling for Mr Lee's name.

The controversy over the Hong Kong booksellers erupted last year when the five men associated with a Hong Kong store that had specialised in gossipy books about China's leaders, including President Xi Jinping, disappeared. Such books are banned on the mainland, but legal in Hong Kong.

China has denied wrongdoing.

Chinese Foreign Ministry spokesman Geng Shuang said China was "resolutely opposed" to Britain's report, and that it could not accept its "unfounded criticism" of China.

He said Hong Kong people enjoyed full rights and freedom under law. "Hong Kong is China's domestic affair. Foreign countries have no right to interfere," Mr Geng told a daily news briefing in Beijing. "We demand that Britain be discreet with its words and stop interfering in Hong Kong affairs."

Hong Kong's government also rejected the report, saying foreign governments should not interfere in its affairs.