Inmates at a high-security jail have been segregated from the general prison population for as long as two years, according to a report.
A watchdog warned the regime at HMP Whitemoor was "not sufficient to prevent psychological deterioration" among those who had been held away from the main wings for prolonged periods.
Segregated inmates could only have a shower and make a phone call every other day, while the "caged" exercise yards were "grim", inspectors found.
Segregation generally involves a prisoner being removed from association with other inmates and can be used for disciplinary reasons or for the individual's own protection.
Inspectors found the segregation unit at Whitemoor in Cambridgeshire was nearly full, housing 28 inmates out of a capacity of 30.
The average length of stay over the previous four months had been about 28 days, according to the assessment from HM Inspectorate of Prisons (HMIP).
But it said the Independent Monitoring Board reported that in December eleven men had been held in segregation for between 11 and 25 months.
And during its inspection in March, HMIP found eight prisoners who had been segregated for more than six months.
In total the prison reported that about 71 inmates had been segregated in the six months prior to the inspection, which was down from 107 recorded at the previous assessment.
Relationships between staff and prisoners in the segregation unit had improved, but the regime was "impoverished", the inspectorate found.
The report said: "While we recognised again the complexity of the men held in the unit, we did not believe that care planning to address their needs was adequate.
"We also felt that the regime offered was not sufficient to prevent psychological deterioration among those who had been segregated for prolonged periods of time."
Some men with "persistently challenging behaviour" were held in the unit while others who were not segregated under prison rules were refusing to relocate back to the normal location, the report said.
Chief Inspector of Prisons Peter Clarke said: "Our overriding concern was about the small but significant number of men in the segregation unit for long periods, and we considered that this needed urgent attention."
Whitemoor is one of five high security "dispersal" prisons in England and Wales, and held 431 adult men at the time of the inspection visit.
The jail continued to hold a disproportionate number of Muslim men, according to the report.
It said Muslims made up 42% and Christians 41% of the population, adding that "the dynamics between the groups could be complex".
Overall, inspectors were "heartened" by what they found at Whitemoor.
Mr Clarke said: "For the vast majority, it was a generally safe prison, conditions were reasonable and relationships with staff had improved."
Michael Spurr, Chief Executive of HM Prison & Probation Service, said: "I'm pleased that the Chief Inspector has commended the work being done at Whitemoor. The Governor and staff deserve real credit for what has been achieved in difficult circumstances."
Work is already in hand to provide more support for men held in long-term segregation, to assist them in returning to the main prison population as soon as it safe to do so, Mr Spurr added.