TV presenter Michael Mosley, known for popularising the 5:2 diet, has gone missing while on holiday on the Greek island of Symi.
A search and rescue operation is under way, including a police dog and a drone searching hard to reach areas.
A helicopter has been deployed from Athens to assist efforts.
Greek Police said Mr Mosley left his wife on the beach and set off on a walk to the centre of the island on Wednesday.
His phone was found in the place he was staying with his wife, who reported him missing, a police spokesperson told BBC News.
The 67-year-old broadcaster is well known for programmes including the BBC series Trust Me, I’m A Doctor and appearances on BBC’s The One Show and ITV’s This Morning.
After officers on the island were unable to find Mr Mosley, they requested help from the Greek fire department in Athens. Firefighters arrived in Symi from nearby Rhodes at 14:00 local time (12:00 BST) on Thursday.
Firefighters, volunteers and police officers are looking for him alongside a police dog, while a drone has also been deployed to help the search.
Officers are also searching CCTV footage for any sign of Mr Mosley.
The rescue operation is focusing on the Pedi area of Symi after a woman saw him there on Wednesday, the island’s deputy mayor Ilias Chaskas told BBC News.
He said the area is considered dangerous, but noted the woman had seen Mr Mosley on the road in a safer part of Pedi.
The island’s mayor, Eleftherios Papakalodoukas, said firefighters carrying out the search told him they believed it was “impossible” Mr Mosley was still there.
“It is a very small, controlled area, full of people. So if something happened to him there, we would have found him by now,” he told BBC News.
Mr Papakalodoukas said he believed it was likely Mr Mosley either “followed another path” or had fallen into the sea.