The brother of a former military bomb disposal expert who is believed to have been murdered said his family have been left devastated by his disappearance.
Mark Manning, 54, was last seen by a friend who dropped him off at Worthing railway station on April 19 and appears to have "vanished", Sussex Police said.
The father of one, who is well travelled and worked for the Mines Advisory Group (MAG), a charity that removes landmines and other dangers in some of the world's poorest countries, was reported missing by his family three days later.
Today Russell Manning, supported by family liaison officer Pc Jo Pither, told a press conference in Worthing, West Sussex, that his brother's disappearance was "completely out of character".
He said: "We are desperate to hear from him or anyone who knows where he is.
"This is completely out of character as he is close to his family and has always kept in touch, even when he has been working in a war zone.
"This is why we are so worried because he has never done this before.
"Mark was well travelled and was in the Army, and then worked as a fisherman.
"Since the early 1990s he has been involved in humanitarian aid and mine clearance worldwide for the Mines Advisory Board.
"This has come as a great shock to all of us and we are devastated."
Appealing to his brother directly, Mr Manning said: "Mark, if you can see or hear this please let us know that you are ok any way that you can.
"We love and miss you."
He continued: "Anyone who knows where Mark is, has seen him, or has any information, even if you think it is insignificant, please call Sussex Police."
Mr Manning lived in a flat on his own in Brighton Road, Lancing, West Sussex, but was very close to his family, particularly his son, a police spokeswoman said.
She said he had missed meetings with family members and had not been in touch with his son on Father's Day, which he had always done no matter where he was in the world.
Detective Chief Inspector Mike Ashcroft said: "Although we can't categorically prove that Mark is dead at this time, we are treating this as seriously as we can.
"His sudden disappearance is completely out of character.
"He is close to his family but they have not heard from him at all. He hasn't used his mobile phone, or carried out any financial transactions, hasn't turned up for pre-arranged appointments and meetings and there has no been no reported sightings of him.
"There were no signs that he was depressed or may wish to harm himself.
He seems to have vanished.
"His family are desperate to know what has become of him and they are being supported by a specially trained police officer.
"We want to hear from anyone who may have seen Mark or has any information about where he maybe."
Mr Manning is white, 5ft 9in, stocky, clean shaven, with a tanned complexion. He has blue eyes and is bald.
Anyone with information should call Sussex Police on 101 or email firstname.lastname@example.org, quoting Operation Light, or Crimestoppers on 0800 555111.