The U.K. has accused Chinese authorities of torturing a former employee of the British Consulate in Hong Kong, a move likely to further damage relations strained in recent months by London's gestures of support for pro-democracy protesters in its former colony.

The intervention comes after Simon Cheng — a Hong Kong resident who worked for the consulate's business development team before he went missing in mainland China for 15 days in August — said on Wednesday he was beaten and forced to confess while detained by Chinese agents, who pressed him for information on participants in the city's protests.

"Simon Cheng was a valued member of our team," U.K. Foreign Secretary Dominic Raab said in a statement. "We were shocked and appalled by the mistreatment he suffered while in Chinese detention, which amounts to torture." Raab said he had summoned the Chinese ambassador to demand an investigation into the "brutal and disgraceful treatment of Simon in violation of China's international obligations."