An employee at the UK’s consulate in Hong Kong has been detained by mainland Chinese authorities on his way back to the city, his girlfriend has said.

Simon Cheng, 28, was returning from a trip in Shenzhen to his native Hong Kong on 8 August when his girlfriend, Li, stopped receiving communications from him.

A spokesman for the UK’s Foreign and Commonwealth Office said: “We are extremely concerned by reports that a member of our team has been detained returning to Hong Kong from Shenzhen … We are providing support to his family and seeking further information from authorities in Guangdong province and Hong Kong.”

Li said Cheng had messaged her just before he went silent. “Ready to pass through the border … pray for me,” he had written.

More than 10 days later, Li and Cheng’s family have not been able to get in touch with him. Li said Hong Kong immigration authorities told her Cheng had been placed under “administrative detention” in mainland China in an unknown location and for unknown reasons.

The detention of Cheng, who works in the British consulate as a trade and investment officer for Scottish Development International, comes amid more than two months of pro-democracy protests in Hong Kong that have threatened Beijing’s authority over the city.

Hundreds of thousands of protesters dressed in black from all ages rallying in Victoria Park in Hong Kong on Sunday.

Cheng’s detention was first reported by the Hong Kong news site HK01. According to Li, he regularly travels to mainland China for meetings and had gone there most recently for work.

Hong Kong immigration officials said in an emailed statement that they have contacted the city’s liaison office in Guangdong province as well as the Hong Kong government “to understand the situation” and provide further assistance to the family.

Hong Kong has been rocked by mass protests triggered by a bill that would allow the extradition of suspects to mainland China. Cheng’s case further underlines fears of a lack of transparency and fairness in the mainland Chinese judicial system and the possibility that Hongkongers may be detained for political reasons.