Cheng fell out of contact as he tried to pass through Chinese immigration that evening and has since been detained in the mainland for unknown reasons, she said, citing information she received from Hong Kong immigration officials. A friend of Cheng’s who is in touch with his family confirmed to The Washington Post that Cheng has not been home since that date.
Beijing has accused Washington and London of fomenting protests that have convulsed Hong Kong over the summer. Canada has criticized China for holding two of its citizens, including a former diplomat, as political hostages in retaliation for the arrest in Vancouver of Meng Wanzhou, an executive at Chinese tech giant Huawei who is wanted in the United States on charges related to alleged violations of sanctions on Iran.
There is no indication that Cheng, a Hong Kong permanent resident, was traveling under a British diplomatic passport when he vanished. His public social media profiles show that he began working at the consulate in December 2017 after receiving a master’s degree from the London School of Economics.
“We are extremely concerned by reports that a member of our team has been detained returning to Hong Kong from Shenzhen,” a British Foreign Office representative said in a statement Tuesday. “We are providing support to his family and seeking further information from authorities in Guangdong Province and Hong Kong.”
Hong Kong’s Immigration Department said it was assisting Cheng’s family and had been in touch with mainland-based Hong Kong officials about the case.
Meanwhile, at a regular briefing in Beijing, Chinese Foreign Ministry spokesman Geng Shuang denied knowledge of the incident.
Tommy Cheung, a former student leader and a friend of Cheng’s, said he is aware of other Hong Kong residents detained in recent weeks in the mainland.
Cheng wrote “Passing through” and “pray for me” in a phone text to his girlfriend as he approached Chinese immigration at the West Kowloon high-speed rail station in Hong Kong, according to HK01, citing images provided by his girlfriend. Under an arrangement between Hong Kong and mainland authorities, cross-border travelers pass through immigration inside the West Kowloon terminus.
Kerry Brown, a former British diplomat who heads the Lau China Institute at King's College London, said Cheng appeared to be a local hire who was not covered under certain diplomatic protocols and was, therefore, more vulnerable than diplomats posted overseas.
“Far too frequently those who work in this situation are regarded by the Chinese as having divided loyalties. They are the ones who have pressure put on them, as pawns in a game,” Brown said.
China could be sending a message to London, he added, noting: “It’s a soft way of not doing a more extreme thing of expelling diplomats.”
Details of Cheng’s disappearance emerged as political tensions continued to simmer in Hong Kong following another huge march over the weekend.
A proposal to allow certain suspects to be extradited from Hong Kong to face trial in the mainland’s Communist Party-controlled courts triggered the weeks of demonstrations in the city, where many viewed the now-shelved plan as a ploy by local leaders, acting in concert with Beijing, to erode Hong Kong’s autonomy and comparative freedoms.
Hong Kong is governed under a “one country, two systems” arrangement within China, under which the territory is supposed to enjoy a high degree of autonomy for 50 years following its return to Chinese sovereignty in 1997.
Activists say there have been growing signs of Beijing’s encroaching influence over Hong Kong, including tightening electoral restrictions, the expulsion of a foreign journalist, abduction of dissidents and the controversial opening of the train station where Cheng disappeared.
Arrangements at the station, which opened in September, allow for mainland police to operate and enforce mainland law within the terminus — the first time such measures have been permitted in Hong Kong. A report by the official Xinhua News Agency last year said that any traveler entering or leaving the station would be considered to be in the mainland and subject to Chinese law.
After the weekend protests, Hong Kong leader Carrie Lam pledged Tuesday to hold talks with people from across the political spectrum but did not appear to budge on demonstrators’ key demands, including the full withdrawal of the extradition bill and an independent investigation into police conduct during the protests.
For weeks, China’s government has framed the protests as a “color revolution” instigated by the United States and Britain. China publicized the personal details and photo of an American diplomat based in Hong Kong this month, in an apparent effort to portray her as a hostile intelligence officer, drawing rebuke from the State Department.
Tensions have been particularly high at the border where a river separates Shenzhen from Hong Kong.
In recent weeks, Chinese immigration authorities have demanded that travelers unlock their smartphones to show their message records and photo albums for inspection. Foreign journalists also have come under heightened scrutiny.
The Vienna Convention governs diplomatic relations between states. An earlier version of this article incorrectly referred to the Geneva Conventions.
Shibani Mahtani contributed to this report.