Hundreds of alleged or convicted sex offenders have been placed in a prison to help tackle high levels of violence, according to a watchdog report.
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."
Surging levels of violence have affected much of the prisons estate in England and Wales.
An inspection report published earlier this year on HMP Whatton, where the population is made up exclusively of sex offenders, found it was an "overwhelmingly safe" jail with comparatively little violence or anti-social behaviour.
The Ministry of Justice (MoJ) 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 HM Prison and Probation Service, 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 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."
Copyright (c) Press Association Ltd. 2017, All Rights Reserved. Picture (c) Anthony Devlin / PA Wire.