Prisoners in Maghaberry jail in the North who opted for segregation because of fears for their safety claim the regime is far more restrictive than they were promised.
One of them is taking a High Court action challenging the way the Prison Service is operating the so-called Compact Scheme, introduced in the wake of the Steele Report into segregation.
Mr Patrick Leonard, a remand prisoner who is one of 25 segregated republicans, is applying for a judicial review next week.
In papers lodged in court, Mr Leonard claimed he is often locked up for 21 hours a day, and that facilities for education and rehabilitation are extremely limited.
He said he also has concerns about his safety arising out of contact with loyalist paramilitary prisoners when going to and from family and legal visits.
His solicitor, Mr Paul Pierce, of Kevin Winters and Co, said: "Our concern is that any prisoner applying for segregation because of fear of assault is being placed in a punitive regime where their access to association, education, the gym and other facilities is restricted."
Mr Leonard said in a sworn affidavit: "My experience regarding my freedom of association has been markedly different to that set out in the notice outlining the conditions of the Compact Scheme.
"Since I was moved to Roe House last March, I have been subject to an intense controlled-movement regime where only three prisoners at most are allowed out of their cell at any one time.
"This level of control is excessive and disproportionate. I do not accept that I need to be effectively locked up for 21 hours a day by myself, and could at the very least be allowed to communicate with other prisoners in my house."
Regarding fears for his safety, Mr Leonard said the Prison Service had informed him that there could be no cast-iron guarantees.
"This contrasts poorly with the rigid, inflexible and excessive level of control when I am in Roe House, and where the risk is minimal," he said.