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".
The report's key findings include:
- The level of self-harm and self-inflicted deaths was high. Typically there were about 32 incidents of self-harm each month, and between January 2010 and the inspection in February 2011 there had been 11 deaths in custody, of which four were apparently self-inflicted
- At best prisoners were locked in their cells for 16.5 hours a day; at worst they were locked up for 22 hours a day
- Poor staff/prisoner relationships; a lack of a predictable regime and insufficient activity all contributed to feelings of isolation and alienation that might have led to self-harming behaviour
- Prisoners with specific individual needs were particularly disadvantaged. One prisoner with a disability who had been remanded for three months told inspectors he hadn't had a shower in that time
- Provision for foreign national prisoners was poor, some being held beyond their sentence - one for three years
- Black and minority ethnic prisoners were disadvantaged in significant areas of the prison
However, the report did acknowledge that the prison had good training opportunities and some good resettlement services.
General and mental health services fared well, and there were few complaints about the food.
The report has been greeted with dismay by prison campaigners.
Howard League for Penal Reform chief executive Frances Crook said: "Such abuse hampers safe return to the community and puts victims at risk.
"If Wandsworth Prison is unable to offer basic facilities such as a shower each day, how much rehabilitation and work to reduce re-offending do we think is going on?"
National Offender Management Service chief executive Michael Spurr acknowledged the poor report, but said a "robust action plan is in place to address the recommendations in the report and managers and staff at the prison are in no doubt that they must improve performance."