The family of a prisoner who hanged himself were told of his death by inmates using an illicit mobile phone.
Rocky Stenning, 26, who had a history of suicide attempts, died at HMP Chelmsford nine days after beginning a nine-year jail term in July 2018.
An ombudsman report said prison staff failed to address his suicide and self-harm risks, adding that such concerns had been raised with the prison before.
The Ministry of Justice said it had improved training and added staff.
The ombudsman concluded Mr Stenning, a mixed martial artist from Tilbury in Essex, did not receive care equal to the standards he would have received outside of prison.
HMP Chelmsford was placed in special measures in June 2018, the month before his death.
Mr Stenning, who had bipolar disorder, had been sectioned and held in a secure unit prior to being sentenced on 10 July.
Despite a history of mental health problems, cannabis use, suicide attempts and having received a long sentence, staff did not consider him to need additional support.
They later noticed he was “feeling overwhelmed” and, on 19 July, transferred him to a quieter residential wing. Three hours later he was found dead.
Prison and Probations Ombudsman Sue McAllister said she was “very concerned” staff had not identified Mr Stenning as being at risk.
There were also shortcomings in the emergency response, with nurses unable to correctly operate oxygen cylinders or take vital readings, she said.
Before prison staff were able to contact Mr Stenning’s family, his sister was told of his death by inmates – which the ombudsman called “very distressing”.
He had been unable to call his family during his sentence as an “oversight” had barred him from accessing the telephone system.
A spokesman for the Ministry of Justice said it had provided “specialist training for staff to help them identify, monitor and support vulnerable offenders”.
It had also recruited an extra 60 prison officers in the past year, she said.
Mr Stenning was the 12th person to die at HMP Chelmsford since July 2015, and the seventh to take his own life. Four more prisoners have died since.