The case has sparked fury from lawmakers and activists in semi-autonomous Hong Kong, adding to growing unease that freedoms in the city are being eroded.
The statement from the Foreign and Commonwealth Office did not specify which of the missing men is British, but it is believed to be Lee Bo, 65, the latest bookseller to go missing.
Lee disappeared last week and was last seen in Hong Kong.
The five missing men all work for Hong Kong-based publishing house Mighty Current, known for books critical of China.
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"We can confirm that one of the individuals is a British citizen and we have urgently requested the Hong Kong and mainland authorities' assistance in ascertaining this individual's welfare and whereabouts."
The FCO urged the Hong Kong government to "honour its commitment" to press freedom.
It added that it hoped Chinese authorities would "continue to make every effort to ensure that the environment in which the media and publishers operate in the Hong Kong... supports full and frank reporting".
Hong Kong was handed back to Beijing by Britain in 1997 and enjoys freedoms unseen on the mainland, and Chinese law enforcers have no right to operate in the city.