The UK Foreign Office has expressed “extreme concern” over reports that a Hong Kong consulate employee had been detained at the Chinese border.
Media reports said Simon Cheng, who is thought to be from Hong Kong, is believed to have gone missing on 8 August.
A FCO statement said it was “seeking further information from authorities in Guangdong Province and Hong Kong”.
The British embassy in Beijing is providing support to the family.
An FCO spokesperson said: “We are concerned by reports that a member of our team has been detained while returning to Hong Kong from Shenzhen.”
Local outlet HKFP reports that Mr Cheng is a trade and investment officer at the Scottish Development International section of the consulate, and had travelled to a business event in Shenzhen on 8 August via the Lo Wu immigration control point.
His girlfriend told news site HK01 he had planned to travel home by train the same day, but did not return.
Travellers have described heightened security measures at the border between Hong Kong and China, as Beijing looks to curb anti-government protests in Hong Kong which it has called “close to terrorism”.
Recent travellers have reported that everyone passing through the border from Hong Kong into mainland China was subject to police checks on the mainland side, where officers took people’s phones and scrutinised their photos and videos.
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The protests, now entering their third month, were sparked by a controversial extradition bill which has since been suspended.
It has now grown into a broader movement calling for democratic reform in Hong Kong, and an investigation into alleged police brutality against protesters.
Organisers say 1.7 million people attended a pro-democracy rally in central Hong Kong on Sunday.
Police put the figure much lower at 128,000, counting only those at an officially sanctioned rally in the city’s Victoria Park.
Some observers believe Beijing’s hardening rhetoric is a sign China is losing patience with the protesters and raises the likelihood of a direct intervention by Beijing.
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Analysts say this remains unlikely, despite the thousands of armed police stationed across the border in Shenzhen.
On Monday, Twitter and Facebook said they had taken steps to block what they described as a state-backed Chinese misinformation campaign on social networks.
Twitter said it removed 936 accounts it said were being used to “sow political discord in Hong Kong”.
The network said the accounts originated in mainland China and were part of a coordinated attempt to undermine the “legitimacy and political positions of the protest movement”.
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Facebook said it had removed “seven Pages, three Groups and five Facebook accounts”.
“They frequently posted about local political news and issues including topics like the ongoing protests in Hong Kong,” said Nathaniel Gleicher, Facebook’s head of cybersecurity policy.
“Although the people behind this activity attempted to conceal their identities, our investigation found links to individuals associated with the Chinese government.”