People sit on the beach of Agios Nikolaos, from where British doctor and television presenter Michael Mosley is believed to have set out, on the southeastern Aegean Sea island of Symi, Greece, Friday, June 7, 2024. ANTONIS MYSTILOGLOU/AP

An ongoing major search and rescue operation on the Greek island of Symi has still not located British doctor and television presenter Michael Mosley, who went missing on Wednesday afternoon after reportedly going for a walk, Greek police say.

The coast guard, police and fire department were involved in the search, with a helicopter, drones and a police sniffer dog deployed, police said Friday. They have found no trace of the 67-year-old who had been vacationing on the island.

The search was focusing on the area between Agios Nikolaos, the coastal area from where Mosely apparently set off on his walk, and the island's main town of Symi. Authorities had also looked into whether he might have left the island altogether, but found no indication that anyone had traveled under his name.

The Reuters news agency reports Mosley's mobile phone was not with him.

"Probably he fell into the sea and washed away, or something else has happened," Symi Mayor Lefteris Papakalodoukas told the news agency.

Police were also looking into reports he spoke to locals during his walk.

Mosley is well-known in Britain for his regular appearances on television and radio and for his column in the Daily Mail newspaper. He is known outside the U.K. for his 2013 book "The Fast Diet," which he co-authored with journalist Mimi Spencer. The "5:2 diet" set out how people can lose weight fast by minimizing their calorie intake for two days in a week while eating healthily on the other five.

He has subsequently introduced the Fast 800 diet, a rapid weight loss program, and has made a number of films about diet and exercise.

Mosley has often pushed his body to extreme lengths to see the effects of his diets and also lived with tapeworms in his guts for six weeks for the BBC documentary "Infested! Living With Parasites."

Worried friends and colleagues expressed their shock over Mosley's disappearance.

"This shocking news focusses our minds on Michael's wife Clare and their sons," said Roy Taylor, a professor of medicine and metabolism at Newcastle University who co-authored "The 8-Week Blood Sugar Diet" with Mosley. "Our thoughts are with them in this terrible time of uncertainty, hoping that he will be found safely."

Britain's Foreign Office said it was in contact with Greek authorities over the search.

Mosley was nominated in 2022 for an Emmy for his executive producer role on the BBC science documentary "The Human Face," which was presented by Monty Python star John Cleese and which featured a raft of famous faces including Elizabeth Hurley, Pierce Brosnan and David Attenborough.

Mosley has four children with his wife Clare Bailey Mosley, who is also a doctor, author and health columnist.