He had written on Twitter and his website before his departure that he may not be heard from for some time, but his family now fear for his safety.

His agent, Joanna Sarsby, told the Daily Mail: "His wife Lenka has not heard from him. She is very worried. He would never miss something like the Hong Kong talk unless something had happened."

Allen, a father of three, had been attempting to track down the Yaifo tribe, which are among the few remaining tribes left on Earth not to have contact with the outside world.