A Foreign Office employee detained in Shenzen has been held for violating Chinese law, according to the nation's foreign ministry.
Spokesman Geng Shuang confirmed this in a news briefing on Wednesday morning and said Mr Cheng was held for violating public security management regulations.
Mr Cheng did not return to work in Hong Kong on August 9 after visiting the neighbouring mainland city of Shenzhen the previous day, according to his family.
A previous statement from the Foreign and Commonwealth Office said it was "extremely concerned" about Mr Cheng going missing.
After confirming Mr Cheng's detention, Mr Shuang also hit out at Britain for commenting on the situation.
He said as Mr Cheng is a Chinese situation it is an internal affair for the country
He added: "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.
"We request they stop making these irresponsible statements, stop meddling in Hong Kong's affairs and stop interfering in China's internal affairs."
A previous Facebook post from his family, detailing his disappearance, said: "He travelled back to Hong Kong by high-speed rail on the evening of the same day, and sent messages to his girlfriend via WhatsApp and Wechat while he was about to pass customs at about 10 pm. We lost contact with him since then."
They also spoke of being "worried sick" and feeling "helpless" over the situation.
The Standard has contacted the Foreign Office for comment.