Two severely disabled prisoners were found crammed into a Victorian cell designed for one for nearly 24 hours a day during an inspection of an "overcrowded" and "deteriorated" jail.
The two older inmates at HMP Winchester in Hampshire had not showered for months and relied on other prisoners to take them their meals, HM Inspectorate of Prisons (HMIP) found.
Elsewhere, one third of the 680 prisoners told inspectors illegal drugs were "easily available", while one in 10 developed a drug habit within the jail walls.
The category B prison, partly built in the 19th century, has deteriorated sharply and is now of serious concern, Chief Inspector of Prisons Nick Hardwick said.
"Until shortly before the inspection, HMP Winchester was neglected and drifting," Mr Hardwick said. "There had been pockets of good practice and, although many staff did their best, their efforts were often haphazard, inconsistent and badly co-ordinated."
In a damning report, the inspectorate said that more than half of prisoners had felt unsafe at some time, while there was insufficient action to tackle violence. Inspectors witnessed "foul abuse" towards vulnerable prisoners in the exercise yard and processes for locating prisoners on to the vulnerable wing were unclear.
Almost four out of five vulnerable prisoners felt unsafe at some time, while "inappropriate sexual images" of naked women were found on the vulnerable prisoner wing.
Staff were unaware that the two older and disabled prisoners could not use the shower on their landing as it was not adapted accordingly. The men had a small window in their cell that was painted over so they had to choose between keeping out the cold or shutting out the light, the report added.
National Offender Management Service chief executive Michael Spurr said: "I accept that performance at Winchester has fallen short of the standards expected. As the chief inspector acknowledges, the new governor has begun to tackle the deficiencies and I am confident that these will be properly addressed and rectified.
"In particular, concerns about levels of violence and first night procedures are being robustly tackled - and we are urgently putting enhanced measures in place to address the supply and circulation of illicit drugs."