Prison staff should have attempted to resuscitate a paedophile maths teacher who died after choking on his breakfast in a prison wing, a report said.
John McKno, 74, died at HMP Norwich in September 2019 and an inquest jury last month found "no evidence... that CPR would have prevented" his death.
But the Prisons and Probation Ombudsman (PPO) said not attempting resuscitation "was not in line with best practice".
The Prison Service said staff had since had extra medical emergency training.
McKno, formerly of Alby in Norfolk, was jailed for 14 years in 2016 for abusing five pupils at boarding schools in Suffolk, Devon and Worcestershire in the 1970s and 1980s.
He worked at Beam College in Great Torrington in Devon, St Michael's College in Tenbury Wells and Kesgrave Hall School near Ipswich.
One pupil at the latter school recalled how he slept in fear with "one eye open".
In a report released after the inquest, ombudsman Sue McAllister said that his care was "at least equivalent to that he could have expected to receive in the community", but she was "concerned about the handling of the incident that led to Mr McKno's death".
Her report said that a healthcare assistant saw McKno turn blue and called for assistance on the radio but "failed to call a medical emergency code", which delayed the emergency response.
A nurse had her radio turned down so "did not initially hear the call for assistance" and a healthcare assistant with her "did not have a radio because she said she had a bad back", which meant they "did not respond as quickly as they should have done", Ms McAllister said.
She added: "The lead nurse directed that staff should not start CPR because Mr McKno had a [do not attempt resuscitation] order in place. This was not the correct decision given that Mr McKno had been choking."
A Prison Service spokeswoman said: "While the ombudsman found Mr McKno received a good standard of care at Norwich, since his death in 2019 staff have received extra training on dealing with medical emergencies."
Dr Peter Taylor, chief medical officer at Virgin Care, which provides healthcare at the prison, said: "Firstly, we would like to express our sympathies regarding the death of Mr McKno.
"We carried out a thorough internal investigation of the circumstances and - as was found at the recent inquest - it was not clear that any actions would have changed the sad outcome."
He said they would work to "ensure that recommendations" from the PPO report were implemented.