An employee of Britain's consulate in Hong Kong who went missing earlier this month is being held in China, Beijing confirmed Wednesday.
The incident comes as relations between Britain and China have become strained over what Beijing calls London's "interference" in pro-democracy protests that have wracked Hong Kong for three months.
Chinese foreign ministry spokesman Geng Shuang told a regular press briefing the detained man had been "placed in administrative detention for 15 days as punishment" by Shenzhen police for breaking a public security law.
Geng said the employee was from Hong Kong and therefore the issue was an internal matter.
"Let me clarify, this employee is a Hong Kong citizen, he's not a UK citizen, which is also saying he's a Chinese person," Geng said.
The man, named by his family as Simon Cheng, travelled to Shenzhen, a megacity on the China-Hong Kong border, for a one-day business meeting on August 8.
That night, Cheng returned via high-speed train and sent messages to his girlfriend as he was about to go through customs.
"We lost contact with him since then," the family said in a Facebook post.
Geng said the employee had violated the Public Security Administration Punishments Law -- a law with broad scope aimed at "maintaining public order in society" and "safeguarding public security", as well as making sure police and security forces act within the law.
- High tensions -
The ongoing protests have raised fears of a Chinese crackdown.
The unrest was initially triggered by a controversial law that would allow extradition to the mainland, but has since broadened into a call for wider democratic reforms.
Beijing has repeatedly warned Britain -- the former colonial ruler of Hong Kong -- against any "interference" in the protests, which erupted 11 weeks ago and have seen millions of people hit the streets calling for democratic reforms.
"Recently the UK has made many erroneous remarks about Hong Kong," Geng said at the press briefing Wednesday.
"We once again urge the British side to stop gesticulating and fanning flames on the Hong Kong issue."
With Beijing attempting to shape the narrative of the unrest in Hong Kong, Chinese authorities have increased their inspections at the Shenzhen border, including checking the phones and devices of some passengers for photos of the protests.
The mainland metropolis of Shenzhen sits behind China's "Great Firewall" --which restricts access to news and information -- while Hong Kong enjoys liberties unseen on the mainland.
China promised to respect the freedoms in the semi-autonomous territory after its handover from Britain in 1997, including freedom of speech, unfettered access to the internet and an independent judiciary.