A prison's response to the Covid-19 pandemic led to it being "less safe" and "less purposeful", a report found.
Inspectors found "troubling" conditions at HMP Erlestoke, including violence, indiscipline and cases of self-harm.
A scrutiny visit took place last month to assess how conditions had changed, since heavy restrictions were imposed at the start of the pandemic.
A Prison Service spokesman said it had taken "immediate action" to address the issues identified in the report.
Some inmates in the segregation unit were held in cells without running water or toilets for weeks at a time, the report found.
"We are urgently working to identify additional improvements we can make to prisoner safety, and Erlestoke will receive additional staff training and specialist support to help drive down violence," the Prison Service spokesman added.
HM Chief Inspector of Prisons, Peter Clarke, said the response to the pandemic at the category C prison, near Devizes in Wiltshire, had "led to a less safe, less decent and less purposeful prison".
"Although the amount of time prisoners could spend out of their cells had been increased in the early stages of lockdown, during our visit... most prisoners still only received 45-minute sessions in the morning and the afternoon, and an additional half an hour one evening a week," he said.
"Prisoners reported being frustrated about daily delays in the delivery of this limited regime, and about the lack of activity."
Other concerns raised in the report included:
- Antisocial behaviour was being rewarded, and some prisoners were resorting to it to get their needs met
- Despite prisoners being locked up for most of the day, the level of assaults had remained similar to the level before the lockdown
- Broken cell windows with sharp shards of glass, damaged observation panels, blocked toilets and showers that were not working
- Racist graffiti in the prison
- Almost a third of prisoners said it was easy to get drugs in the jail
- Significant amounts - 370 litres since the start of the pandemic - of illicit alcohol was found.
Mr Clarke said it was "a very troubling visit" with some issues "systemic, arising from the apparent inflexibility of the recovery programme".
"Well-led and properly supported local innovation and flexibility are now urgently needed to restore the acceptable treatment and conditions of the prisoners held there," he added.
He said he had raised concerns in a letter to Justice Secretary, Robert Buckland, and had received a response which was "in effect" an action plan to address the issues.