The treatment of inmates at Wandsworth Prison in London was "demeaning, unsafe and fell below what could be classed as decent", a report says.
Chief inspector of prisons Nick Hardwick said he "did not detect sufficient willingness" in the prison to acknowledge and address concerns.
The report says conditions were significantly worse than in June 2009.
The National Offender Management Service said a "robust action plan" was in place.
Mr Hardwick's report followed an unannounced inspection of the prison between 28 February and 4 March which was undertaken because a previous inspection - in June 2009 - had been marred by an attempt to subvert the process by moving "difficult" prisoners between Wandsworth and Pentonville so that they were not present in either prison during the inspection.
Written on behalf of Her Majesty's Inspectorate of Prisons for England and Wales, the report says prisoner safety was now a "matter of serious concern".
CLICK HERE for the full report - source BBC NEWS