An attempt to resuscitate a "clearly dead" inmate jailed for ordering the "cold-blooded execution" of his ex-wife was inappropriate, a report found.
Graham Birchwood, 65, was convicted of murder in 2009 after he paid a friend £30,000 to kill Sharon Birchwood so he could inherit her £475,000 assets.
He died from heart disease at HMP Wayland in Norfolk in September 2019.
A HM Prison Service spokesman said officers had since had "extra training" on dealing with medical emergencies.
Businessman Birchwood, from Epsom in Surrey, had debts of £150,000 and arranged for his friend Paul Cryne to kill Mrs Birchwood, 52, who was registered disabled.
Her body was found "cruelly trussed up" at her home in Ashtead in December 2007 after Cryne carried out the killing with "ruthless efficiency", a court heard.
Jailing Birchwood at Croydon Crown Court in 2009 for life with a minimum term of 32 years, Judge Warwick McKinnon described the murder as "a cold-blooded execution for financial gain".
Birchwood was found dead on his cell floor on 9 September 2019.
A Prisons and Probation Ombudsman (PPO) report said when he was discovered staff "failed to call a medical emergency code which led to a delay in the emergency response".
"While it made no difference to the outcome for Mr Birchwood, any delay in a future medical emergency could be critical," it said.
Staff initiated CPR, before a "more experienced nurse arrived [and] could see that Mr Birchwood had rigor mortis and was clearly dead so he told staff to stop".
The ombudsman called it "disappointing that staff tried to resuscitate Mr Birchwood when he had clearly been dead for some time".
"This was inappropriate," the report added.
A Prison Service spokesman said: "Our condolences remain with the family of Mr Birchwood and since his death officers have been given extra training on how to deal with medical emergencies."