Untrained prison staff failed to take action which could have prevented an inmate's death, an inquest has ruled.
Sean Plumstead, 27, was found hanged in his cell at Winchester prison, Hampshire, on 15 September 2016.
An inquest jury said the father-of-two had showed signs of being suicidal but no action was taken because staff had "inadequate" training over self-harm.
The coroner said untrained agency workers were employed in the prison against national policy.
Mr Plumstead, from Hayling Island, died one month before he was due to be released from a 16-month sentence for assault.
Prison staff took more than 10 minutes to respond after his cellmate pressed a warning bell, the hearing was told.
They restored his pulse but the inmate died three days later in hospital.
The jury said the emergency cell bell (ECB) system was "not fit for purpose" and may have contributed to the death.
In a report to the Prison Service, Coroner Grahame Short said "widespread misuse" by prisoners sometimes led to bells being ignored for more than hour.
'Shocked and distressed'
Mr Short said he was also concerned about the role of agency workers at the prison.
He said: "At least two Carillion staff were employed in prisoner-facing roles... without any training in self-harm / suicide prevention in apparent contradiction to the national policy."
He said Carillion, which declined to take part in the inquest, was "either unaware of or unconcerned with" the issue.
In a statement, Mr Plumstead's mother Lisa Dance said she was "shocked and distressed" at the prison's failures.
The Inquest charity, which campaigns over deaths in custody, called for an "urgent intervention" at HMP Winchester, following "nine self-inflicted deaths in five years".
The Prison Service said in a statement it is now providing extra training for staff at HMP Winchester to better support vulnerable offenders. It added that all new prison officers in England and Wales had received suicide and self-harm prevention training since May.
It added there will also be an independent investigation by the Prisons and Probation Ombudsman.