The “deliberate policy” apparently aimed at producing a “stabilising” effect was flagged up in an assessment of HMP Doncaster following an inspection in July.
HM Inspectorate of Prisons (HMIP) detailed how there had been a change in the profile of the jail’s population.
The number of men on remand for, or convicted of, sex offences had trebled to just over 300 at the time of the inspection.
Chief Inspector of Prisons Peter Clarke said: “I was told that this was a deliberate policy in order to help to stabilise the prison in light of the serious problems with violence that had been identified at the last inspection.”
He warned that the support, offender management and programmes intended to reduce the risk both in custody and on release presented by this population were not present.
Mr Clarke said: “In effect, this large cohort of men was being denied the opportunity to make progress.
“While it is perhaps understandable that, as a matter of policy, it might be decided that a prison should have a particular population profile, this should not be done in such a way that offender management of those prisoners is neglected.”
The report called on HM Prison and Probation Service (HMPPS) to support and resource the jail in developing a comprehensive strategy which “clearly identifies how men convicted of a sexual offence will be offender managed; how their risk of harm will be reduced; how they will progress through their sentence; and how the public will be protected during custody and on release.”
The Ministry of Justice confirmed that Doncaster was chosen as a suitable prison to hold an increased number of sex offenders.
While it is not national policy, the prison is using the move as part of a local strategy to improve stability, the MoJ added.
Michael Spurr, chief executive of HMPPS, said: “The leadership team at Doncaster are managing an increase in sex offenders and have created a specific houseblock dedicated to providing the right regime to support their rehabilitation as part of our wider population strategy to manage sex offenders effectively.”
HMP Doncaster, which is also a young offender institution, is a category B prison near the centre of the South Yorkshire town.
Operated by Serco, it held just over 1,100 adult and young adult males at the time of the inspection.
The inspectorate found levels of violence at the establishment had reduced sharply, but they remained high.
The report said poor behaviour from inmates too often went unchallenged, adding that prisoners “gathered in cells, smoked on the landings, walked around partially clothed and ignored staff instruction without fear of reprimand”.
However, the watchdog also made a number of positive findings and Mr Clarke acknowledged that a “great deal has been achieved” at the prison.
Inspectors found the jail was “more stable” overall compared with the previous assessment in 2015, while living conditions had improved “substantially”.
Serco contract director at HMP&YOI Doncaster Jerry Spencer said: “My entire staff has worked incredibly hard to address the complex challenges the prison faces, many of which are found across the prison estate.
“As the Chief Inspector notes, a great deal has been achieved, but we know there is much more still to do.”
Doncaster Prison was built on the site of Doncaster Power Station, and opened in 1994. Management of the prison was originally contracted out by the Home Office to Premier Prison Services Ltd, a joint venture between Serco and US company Wackenhut Corrections.