A Chinese national working at Britain's Hong Kong consulate has been detained in China's border city of Shenzhen for violating the law, the Chinese foreign ministry said on Wednesday, likely worsening already strained ties between Beijing and London.

Britain has said it is "extremely concerned" by reports that the staff member at the consulate in its former colony had been detained.

Simon Cheng Man-kit did not return to work on Aug. 9 after visiting the neighboring mainland city of Shenzhen the previous day, Hong Kong news website HK01 reported, citing an interview with his girlfriend and family.

Cheng's family confirmed his disappearance in a Facebook post on Tuesday night, saying he traveled from Hong Kong to Shenzhen on the morning of Aug. 8 for a business trip.

Speaking at a daily news briefing in Beijing, Chinese Foreign Ministry spokesman Geng Shuang said Cheng had been detained for 15 days by Shenzhen police for violating public security management regulations, though he gave no details.

It is unclear whether the man possessed a diplomatic passport and Geng said he was a Hong Kong resident.

Cheng is a Chinese citizen and so the matter is entirely a Chinese internal affair, Geng said.

"As for Britain's comments, we've made stern representations to Britain for the series of comments and actions they've made on Hong Kong," he added.

"We request they stop making these irresponsible statements, stop meddling in Hong Kong's affairs and stop interfering in China's internal affairs."

Shenzhen police declined immediate comment.

Hong Kong has been gripped by anti-government protests in for the last three months, with Beijing accusing Britain and other Western countries of meddling in its affairs.

Britain, the United States and other countries have urged China to respect the "one country, two systems" formula under which Hong Kong returned from British to Chinese rule in 1997.