Jason Hamblett, aged 45, was reported missing yesterday after disappearing after he left Doncaster Royal Infirmary, Thorne Road, at around 11.30am.
He had been to visit someone but has not been seen or heard from since.
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South Yorkshire Police said: “Officers are becoming increasingly concerned for his welfare and want to hear from anyone who has seen him.”
Jason is around 6ft 3ins tall, has grey hair and a goatee beard.
He is thought to be wearing blue Nike trainers and a black Adidas raincoat.
Anyone with information on his whereabouts should call South Yorkshire Police on 101 and quote incident number 952 of June 17.