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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."
Mr Ashcroft added: "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 may be."
Mr Manning is white, 5ft 9in tall, clean shaven, with a tanned complexion and is of stocky build. He has blue eyes and is bald.
A former military bomb disposal expert who vanished almost two months ago is feared to have been murdered, detectives said.
Mark Manning, 54, from Lancing, West Sussex, was reported missing on April 22, three days after his family last heard from him.
He worked for the Mines Advisory Group (MAG), which clears landmines and other explosives from conflict zones around the world.
Relatives have said that Mr Manning has never vanished before and described his disappearance as completely out of character.
He has not used his mobile phone, or carried out any financial transactions and has failed to attend appointments and meetings, police said.
A murder inquiry has now been launched by the Surrey and Sussex major crime team as there have been no reported sightings of him.