A London High Court judge reserved judgment after hearing that two armed robbers had been sent "unlawfully and unfairly" to the unit at Woodhill prison, Milton Keynes, Buckinghamshire, which is known as "Alcatraz".
Lawyers for Rifat Mehmet and Sean O'Connor said Woodhill's close supervision centre had been described as a "brutalising environment". The basic regime forces prisoners to use cardboard mattresses and go without books and other personal possessions. All privileges must be earned by good behaviour.
If the inmates are successful in their legal challenge the Prison Service will be forced to conduct an overhaul of how the unit is used and which offenders are referred there.
The conditions have concerned Sir David Ramsbotham, the Chief Inspector of Prisons, and Stephen Shaw, the director of the Prison Reform Trust, who described them as "barren" and "cruel".
Mehmet and O'Connor are serving 27 years and 12 years respectively. The unit also houses notorious prisoners such as the hostage-taker Charles Bronson, who changed his name to that of the American actor, and the kidnapper Michael Sams.
The court was told Mehmet spent 23 hours a day locked in his cell. A psychiatrist had concluded that his continued detention in the unit would "produce psychotic symptoms in the extreme".
The centre, which was opened last February, is combined with two other units at Hull and Durham prisons to provide 53 places for the most violent inmates, with about 30 held at Woodhill.
Outside the High Court, Phil Wheatley, a senior Prison Service official, said the unit was becoming "better and safer".Reuse content