The Foreign Ministry on Wednesday confirmed that an employee of the British Consulate in Hong Kong has been detained on the mainland, but said it was not a diplomatic issue as the person held was Chinese national.
British authorities on Tuesday raised an alarm after consulate worker Simon Cheng failed to return to Hong Kong after going across the border on August 8 to attend a business event.
Answering a question about it, the Foreign Ministry spokesman Geng Shuang said: "The person you mentioned has been placed in administrative detention for 15 days as punishment".
Geng said this was carried out by the Shenzhen police for breaking a public security law.
He said though it was not a foreign affairs matter, because it generated a lot of attention, Beijing authorities have been at pains to get more information from the relevant departments.
"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.
Asked about the concern raised by London, the spokesman said Britain has been interfering in the domestic affairs of China by commenting on Hong Kong events and Beijing had repeatedly asked them to refrain from it.
On Tuesday, Hong Kong police said they haven't got any information about detention of Cheng under the mutual notification mechanism.
Cheng's family had said that he had messaged them, saying he was taking the high-speed rail on his way back to Hong Kong on the evening of August 8.
They said he had messaged his girlfriend on WhatsApp and WeChat just before he was to pass through customs at about 10pm.
"We lost contact with him since then," the family said in a statement posted on Facebook.
The travellers on the express rail go through joint immigration and customs check facility at the West Kowloon station terminus.