The British private investigator detained in China in connection with his work for GlaxoSmithKline has said he feels “cheated” by the UK drugs group as Chinese authorities prepare to try him in secret next month.

In a note dictated from prison and seen by the Financial Times, Peter Humphrey criticised GSK for failing to reveal the full extent of corruption allegations against the company when he was hired last year.

Mr Humphrey, a veteran corporate detective based in China, was recruited by GSK in April 2013 to investigate a former employee suspected of running a “smear campaign” against it. This included emails sent to senior GSK executives containing allegations of bribery by the company, as well as a secretly-filmed sex video involving Mark Reilly, its top manager in China.