Simon Cheng, 28, a trade and investment officer for Scottish Development International, travelled to Shenzhen, a city that borders Hong Kong, on 8 August. He sent messages to his girlfriend as he was about to cross back over the border at about 10pm and has not been heard from since.

A foreign ministry spokesman said on Wednesday that Cheng had violated China’s public security administration punishment law and had been made to serve 15 days of administrative detention in Shenzhen.

The Global Times, a nationalist tabloid owned by the Communist Party mouthpiece People’s Daily, quoted Shenzhen police on Thursday as saying that Cheng was detained “for involvement in prostitution”. It added that the law stipulated that people “who engage in prostitution or visiting prostitutes” should be detained for up to 15 days.

Cheng’s family declined to comment but said “the truth is in people’s hearts” in an online reply to a Guardian inquiry. He is due to be released on Friday.

The Foreign and Commonwealth Office, in a statement, said: “We continue to urgently seek further information about Simon’s case. Neither we nor Simon’s family have been able to speak to him since his detention.”

The Global Times reported that it was Cheng who requested police not to notify his family. In recent years, many Chinese activists have been forced to speak against their will and confess to alleged crimes on state media.

Hong Kong has been roiled by almost three months of pro-democracy protests that have threatened Beijing’s authority over the city. On Sunday more than a million Hong Kong residents defied a police ban and poured into the streets on Sunday in a peaceful march, calling for the withdrawal of a controversial extradition bill and making other demands.

Reports have emerged that many Hong Kong residents have been interrogated upon entering mainland China, taken into rooms, their messages and photos on their phones and computers checked along with documents they carried.

Cheng’s girlfriend, Li, said he had not participated in pro-democracy protests over the past two months or publicly expressed his position on the movement.

China has a long history of arresting dissidents, activists and government critics on trumped up sex or financial crimes.

In 2015 Ou Shaokun, an anti-corruption activist, was given five days of administrative detention for allegedly soliciting a prostitute, although he said he was framed. In 1999, Peng Ming, a prominent pro-democracy activist was given 15 days in custody for allegedly visiting prostitutes. After his release he was sentenced without a trial to 18 months in a police-run “re-education” labour camp, for publishing a book in Hong Kong.