The hunt for the missing Earl of Shaftesbury is now being treated as a murder inquiry, police have confirmed.
The Earl was due to return to the UK from France on 10 November
The 66-year-old has not been seen since early November when he vanished on a trip to the French Riviera. He was due to return to the UK on 10 November.
Two Sussex detectives recently flew to Nice to meet French police searching for the Earl, who has homes in Hove and London as well as the south of France.
Sussex Police confirmed on Tuesday no body has been found.
A statement issued on Tuesday by the earl's eldest son, Lord Ashley, said the family "feared the worst".
"My family and I are extremely concerned about the disappearance of my father," he said.
"We have visited the French investigation in Nice and are convinced that everything that could be done to ascertain my father's fate is being done.
"We remain in close contact with them and Sussex Police and will continue to assist in any way we can."
Family members who accompanied police on the visit to Nice included the earl's ex-wife, Lady Christina Shaftesbury, who lives at the family estate in Wimborne St Giles, Dorset.
Det Sgt Paul Sellings said it had been a very distressing time for Lady Christina and the family
"I've got to pay testament to their resilience," he said.
"She is doing well in the circumstances."
The aristocrat, whose full name is Anthony Ashley-Cooper, last contacted his family on 5 November.
The two detectives, from Sussex Police's major crime branch, spent four days discussing the investigation and exchanging ideas and information with their French counterparts.
During their visit last week, they handed the investigation over to French police, marking the closure of the British side of the inquiry.
The Earl arrived in Nice on 3 November, met his third wife two days later, and checked into the Hilton Hotel on Cannes seafront on 6 November for one night.
He has not been seen since.
Police have described him as being of thin build with thinning grey hair.
They said he often wears large, heavy-framed red and pink glasses and normally wears a large watch and casual clothes.