A 34-year-old prisoner has died at Maghaberry jail, the Northern Ireland Prison Service has confirmed.
This is the third death at the Co Antrim facility this month and brings to six the total number of deaths in the past 12 months - four of whom were suicides.
Acting Prison Service Director General Phil Wragg said: "I would like to extend my sympathy and that of the Northern Ireland Prison Service to the family of the prisoner.
"My thoughts are with them at this difficult time."
The inmate was found dead in his cell on Wednesday morning.
It is understood prison authorities are not treating it as suicide. However, the exact circumstances of his death have yet to be established.
Investigations have been launched by the police, coroner and Prisoner Ombudsman.
Maghaberry, Northern Ireland's only high security prison, accommodates sentenced and remand prisoners and has segregated areas for paramilitary prisoners.
It was ranked among the most dangerous jails in the UK after a damning inspection last year.
Prison authorities were also heavily criticised in a recent Ombudsman report after an inmate inflicted extreme self-harm on himself in June 2014.
Ulster Unionist Party justice spokesman Doug Beattie said this latest death pointed to a crisis within the system.
He said: "Whatever the exact circumstance was, it is simply not acceptable that people who are incarcerated within a state institution are dying on such a regular basis.
"This is the third fatality in the month of November. That fact alone must surely shake away any complacency amongst those in charge of our prison system.
"I repeat what I have said before; there is a crisis in our prison service which deserves the full attention of the Executive, the Minister for Justice, and prison management.
"How many more prisoners have to lose their lives before our Executive realises there is a real crisis within our prisons and our Prison Service that will not go away until decisive and strategic action is taken?"
Earlier this month Stormont ministers announced a review of how vulnerable prisoners are monitored after a number of recent suicides and self-harm incidents.
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