At a daily briefing on Wednesday, foreign ministry spokesperson Geng Shuang said the case was "not a diplomatic issue."
He said the man had violated regulations on "Punishments in Public Order and Security Administration," but gave no further details.
The British foreign ministry has said it is "extremely concerned" about its staff member, identified by local media as Simon Cheng Man-kit, a trade and investment officer at the Scottish Development International section of the consulate.
Reports have said he attended a business event in Shenzhen on 8 August but never returned to neighbouring Hong Kong despite plans to do so the same day.
It is unclear whether the man possessed a diplomatic passport. Mr Geng said he was a Hong Kong resident and not a UK citizen.
Public order and security infractions are generally minor — for instance, fighting on the street — and result in administrative rather than criminal punishments.
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The maximum penalty for such transgressions is 15 days in detention.
The seizure of consular staff of any status or rank is highly unusual. Despite Beijing's declaration of a "golden era" in Sino-British ties, relations have grown tense in recent months amid pro-democracy protests in Hong Kong, a former British colony.
"Regarding the series of words and deeds by the British side recently on Hong Kong-related issues, we have repeatedly lodged solemn complaints with the UK," Mr Geng said, adding that China urges the UK to "stop meddling in Hong Kong affairs."
Hong Kong Senior Police Superintendent Kong Wing-cheung said on Tuesday that Hong Kong law enforcement agencies were not notified about the case by mainland Chinese authorities under a mutual notification arrangement set up for such cases.