John Cole, 65, of Loundsley Green, was reported missing last Monday and his body was found by a cleaner in the toilets of Betfred on Tuesday morning.
Betfred said its policy states toilets should be checked before shops are closed – and the company confirmed an investigation had been launched into the tragic incident.
Mr Cole’s brother Brian, from Brampton, said: “Betfred have questions to answer.
“Quite frankly, the toilets should have been checked before the shop was shut on Monday night.
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“If they had been, he might have been found and taken to hospital for treatment.
“He could still be here today if that had happened – but he’s not.
“If the toilets aren’t being checked before staff go home, it also raises questions about security. Anybody could be in them overnight.”
Brian claimed that when he phoned Betfred’s complaints line he was told ‘there’s not much we can do because your brother is dead’.
Mr Cole’s sister Alison Shirley echoed her brother’s concerns and added: “Family, friends and police were out searching for him for hours on Monday night – but he was in the toilets of Betfred the whole time.
“How could this happen?
“It’s absolutely shocking.”
Mr Cole’s niece Zoe Cole said he was ‘more like a dad than an uncle’.
A life-long resident of Chesterfield, popular Mr Cole – who also has a second brother, Barry – spent 15 years in the Royal Navy.
His family described him as a ‘wonderful man who would do anything for anybody’.
A Betfred spokesman said: “First and foremost our thoughts are with the family and friends of Mr Cole at this very sad time.
“It is our policy that all public areas, including toilets, are thoroughly checked prior to the closing of the shop.
“However, until our investigations have been completed, we cannot comment further.”
A Derbyshire police spokesman added: “There are no suspicious circumstances surrounding the death and a file will be prepared for the coroner.”
Mr Cole died of natural causes.
His funeral will take place at St Bartholomew’s Church at Old Whittington at 11am on Tuesday, November 22.