Explorer Benedict Allen has gone missing after attempting to find an indigenous tribe in Papua New Guinea, it is reported.
The 57-year-old, who has previously recorded series for the BBC and written books on exploration, was on a journey to rediscover the Yaifo tribe.
In a post on his website before setting off titled "I may be some time", Mr Allen said he was looking to meet with the tribe 30 years after discovering them for the first time.
He said: "No outsider has made the journey to visit them since the rather perilous journey I made as a young man three decades ago.
"This would make them the remotest people in Papua New Guinea, and one of the last people on the entire planet who are out-of-contact with our interconnected world."
A helicopter dropped Mr Allen off at Bisorio without a satellite phone, GPS or companion and he was due back in the country's capital Port Moresby on Sunday to travel to Hong Kong.
His agent Joanna Sarsby told the Daily Mail his wife Lenka was "very worried".
She added: "He is a highly experienced explorer, very clever and resourceful and adept at surviving in the most hostile places on Earth, and he would never give up. He may not be a young man any more but he is very fit.
"He was trying to reach the Yaifo people, a very remote and reclusive tribe - possibly headhunters, quite a scary bunch. Goodness knows what has happened.
"I just imagine he might have been taken ill or is lying injured somewhere, perhaps with a broken leg, and maybe being helped by locals. He never takes a phone with him - he believes in living like the locals. For him not to come back is really odd."