Police are looking for Stephen Hamblin, 50, who was reported missing from the Ascot hospital, in London Road, at 7.04am today (Thursday).
Mr Hamblin is white with greying brown hair, 5ft 9ins, with a stocky build and heavily tattooed arms.
He was wearing a round-necked blue T-shirt, blue jeans and possibly a jacket, with black, army-style ankle boots.
Sergeant Charlotte Parker said: "Stephen is prone to violent outbursts and I would advise anyone who sees him not to approach him and contact police immediately
"We are also very concerned for his welfare as he suffers from diabetes.
"I would urge members of the public to contact us as soon as possible if they see him or if they know of his whereabouts."
Anyone with information should contact Sgt Parker via 101.