STAFF at Winchester jail did not realise that a prisoner, who was taken ill in the gym, had suffered a heart attack, from which he died four hours later.
It was Terence Gleadall's first strenuous exercise in 15 years, an inquest heard.
Mr Gleadall, a mechanic, had been jailed for perverting the course of justice after lying to police about service records for a lorry involved in a hit-and-run in Southampton in 2002, in which a pensioner was killed.
The 46-year-old died on October 27th-less than three weeks after starting his sentence.
The inquest heard that Mr Gleadall, from Rotherham, had complained of chest pains and tingling in his fingers.
Prison officer, Mark Beech, thought he had merely over-exerted himself in the jail gym.
"The symptoms weren't of a heart attack but muscular," he said.
The pain eased and Mr Gleadall was able to walk to the jail's health care centre.
Another prison officer, David Barker, said: "He walked there completely unaided. Had he been breathlessness, sweating, showing signs of a heart attack, I would have had him over to the Royal Hampshire County Hospital immediately."
He was placed in a cell with another prisoner, Michael Alden (31), a Southampton tattooist, who, in a statement, said they chatted before Mr Gleadall fell silent.
Soon afterwards, nurse Christine Perkins asked Mr Alden to rouse him and they found then that he had died.
It was about four hours after the initial heart attack.
Home Office pathologist, Debbie Cook, said it was impossible to say whether immediate treatment would have made any difference.
Dr John Stocker, medical officer, said: "It is easy, with hindsight, to say what should have been done. It is interesting to hear Dr Cook's evidence on the extent of the heart attack. It was a significant injury to the heart. It seems, even now, surprising there weren't greater injury symptoms and signs being displayed by Mr Gleadall."
The inquest heard that Mr Gleadall was anxious that he was missing his wife's 40th birthday and that they had spoken on the phone shortly before his death.
No members of Mr Gleadall's family were at the hearing in Winchester on Wednesday when the jury returned a verdict of death by natural causes after Grahame Short, central Hampshire coroner, said no other conclusion was possible.
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