Peter Humphrey had not expected to be caught in the glare of a television camera from China Central Television, the state broadcaster, when he was escorted last August into a small room in a Shanghai detention centre.

Mr Humphrey, a British private investigator hired by GlaxoSmithKline to investigate whether a former Chinese employee was waging a smear campaign against the company, was asked to read a confession into what he was told would be a small police interrogation camera, according to people familiar with his case. Police informed him that it would help his situation.

A month earlier Mr Humphrey and Yu Yingzeng, his American wife and business partner, had been arrested on charges of illegally purchasing private information and operating an illegal business.