Figures obtained by the investigative website, The Detail, show that last year at least 10 prisoners were held in solitary confinement in Maghaberry for over 100 days each, with four inmates held for over a year and in one case a prisoner was held for five years.

The UN considers solitary confinement as the physical isolation of individuals who are in their cells for over 22 hours a day and it has called for a worldwide ban on durations of over 15 days.

Prisoner Ombudsman Tom McGonigle raised concerns that inmates with complex mental health needs are among those being held long term in solitary confinement within Maghaberry prison.

Mr McGonigle said: “These figures are particularly worrying because a lot of prisoners who end up in confinement are already damaged emotionally and psychologically before they go into prison.

“Often that leads to the misbehaviour that brings them into solitary and inevitably it becomes a vicious circle whereby their negative behaviour is reinforced by the time they spend there.

“I believe the solitary confinement figures highlight the wider need for co-operation between the Minister of Health and Minister of Justice to look at how mentally disordered offenders are dealt with.”

He added: “The Care and Supervision Unit was formerly known as the segregation block, and prisoners call it ‘the boards’, but they know what it is. I know what it is. It’s solitary confinement for up to 23 hours a day.

“When in the CSU prisoners are separated from the rest of the prison population and they can’t go to work or take part in education. It is deliberately designed as a disincentive so that prisoners do not aim to go there; it is a very difficult place for prisoners to live.”

Through a series of Freedom of Information requests The Detail obtained a breakdown of the top ten longest stays in the CSU across each of Northern Ireland’s prisons.

The figures show that since 2011 the longest stay by an inmate in Magilligan Prison’s CSU was 138 days.

The longest stay in the CSU in Hydebank Wood prison site - where female prisoners and the young offenders centre are based - was 46 days.

The longest single stay for an inmate in Maghaberry Prison’s CSU was five years.

The Northern Ireland Prison Service (NIPS) said: “Every case is considered on an independent basis and there is a stringent and transparent process in place to manage and review all cases. Prisoners are only held in the Care and Supervision Unit for such a time as it is considered to be absolutely necessary.”

The Detail sought to compare the solitary confinement levels with those in the Republic of Ireland (ROI). Monthly figures from the Irish Prison Service show a decrease of 211 prisoners on 22/23 hour lock up in July 2013 to 74 prisoners in July 2016.