A prison officer was dismissed after she "failed to notice" a sex offender was dead on the floor when unlocking his cell, a report said.
Stephen Maddock, 59, died at HMP Rye Hill on the Northamptonshire and Warwickshire border on 7 December.
A report said an officer opened his cell but did not check on him, "which meant that no-one realised [he] was dead for another half an hour".
HMP Rye Hill's director said it "fully accepted" the report's recommendations.
The Prisons and Probation Ombudsman (PPO) report said Maddock was serving a 16-year sentence, having been convicted of sexual offences in 2015.
Maddock, who was clinically obese, had diabetes and high blood pressure, which he received medication for.
On the morning of 7 December, the report said, officers at the G4S-operated Category B prison for sex offenders carried out roll checks of inmates on Maddock's wing twice before an officer unlocked his cell at 08:00 GMT.
The report said: "About half an hour later, prisoners called for staff after they had found Maddock on his cell floor.
"He had rigor mortis, which indicated that he had been dead for some time."
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A post-mortem examination discovered he died from acute pancreatitis and the ombudsman found the healthcare he received "was of a standard equivalent to that he could have expected to receive in the community".
But the report added the officer, who was later dismissed after a disciplinary investigation, "failed to notice that he was dead on the floor".
"When unlocking a prisoner's cell, the officer is supposed to get a response from the prisoner to satisfy themselves that they are alive and well.
"This did not happen, which meant that no-one realised that Maddock was dead for another half an hour."
The PPO also recommended "that all staff understand what is expected of them when conducting roll checks and that all staff adhere to these expectations".
Peter Small, director at HMP Rye Hill, said: "Mr Maddock's family and friends remain in our thoughts at this difficult time.
"We have fully accepted the recommendations made by the PPO; staff conducting welfare checks do so with regular managerial supervision, and daily reports are issued to the deputy director."