The detainee unit at HMP Long Lartin holds a small number of individuals suspected but not charged with involvement in international terrorism and held under immigration or extradition law.
Seven detainees were being held at the unit, which can hold up to 19 people, at the time of the inspection in April.
At least two foreign nationals have been held at the facility for more than 11 years awaiting deportation or extradition while one British national Babar Ahmad, who is fighting extradition to the US, had been detained there for seven years.
The prisons inspectorate had previously raised concerns about the “holding a small number of detainees, who already inhabit a kind of legal limbo, in a severely restricted environment for a potentially indefinite period.”
But Chief Inspector Nick Hardwick said that in spite of this previous warning he was concerned to find that “the detainees were no longer able to mix with the wider prison population.”
These restrictions had apparently been made on security grounds, he said.
“Although the rationale appeared obscure as sentenced terrorists faced no such restriction in the main prison and not all detainees posed the same level of risk.”
Mr Hardwick said that the prison governor had recently “offered to allow some mixing” but only with vulnerable prisoners such as sex offenders, informers and other inmates at risk from the general prison population.
“This had been rejected by the detainees as stigmatising,” he said. He called for a review of the situation.
The inspectors also expressed concern that a number of previous recommendations had not been acted on.
“In particular there were still no operating standards specific to category A detainees and we were not assured that additional restrictions on movement around the unit and on the regime imposed since our previous inspection were appropriate and proportionate,” Mr Hardwick said.
The risks to the mental and physical health of detainees of such “lengthy, ill-defined and isolated confinement” are significant, he concluded.
He added that “the Prison Service needs to ensure a better balance is struck between security and humane care and between separation and integration.”
The unit was clean but basic, inspectors found. Detainees felt safe and relations with staff were polite and professional, but too little attention was paid to the “uniquely isolated and uncertain position” of the detainees.
National Offender Management Service chief executive officer Michael Spurr said: “There is an inevitable balance to be struck between legitimate security concerns and integration with the general population at HMP Long Lartin.
“The governor will continue to monitor the situation to ensure that the right balance is maintained.”