Two elderly disabled men were locked up together at Winchester prison for almost 24 hours a day, without easy access to showers.
Inspectors said the men tried to wash in their cell but nearby showers were not adapted for disabled people.
A damning HM Inspectorate of Prisons report revealed the inmates' plight among a raft of "serious concerns".
It said the Victorian prison "deteriorated sharply" but new governor David Rogers was tackling problems.
The men, one in his mid-60s and the other in his 80s, could not work and were confined to their cell for 23-and-a-half hours a day and relied on fellow inmates to bring them meals.
The cell, meant for one prisoner, had only one small window which was painted over.
Drugs and violence
Mr Rogers said: "There were issues around them because we don't have disabled showering facilities on the wing.
"However there were disabled facilities available for them in an adjacent wing.
"They chose not to engage with us or use those. We should have tried to engage with them better".
HMP Winchester was built in 1846 and houses
category B and C offenders.
A separate annexe was built in 1963 and housed
young offenders. In 1995 this became West Hill
Prison for adult women.
In 2005 the annexe became an category C adult
male training unit.
The prison's C wing was fully refurbished
in 2008 and has a variety of uses including
induction, first night stay and detoxification.
One of the inmates has now been released. The other has moved to another prison.
Meanwhile more than half of prisoners reported feeling unsafe and inspectors criticised the handling of violent incidents.
One third of the 680 prisoners said drugs were easily available and one-in-10 reported developing a drug problem in jail.
However the experience of prisoners at category C annexe West Hill was "much better".
Mr Rogers said: "Winchester fell well below the standards expected and I'm not going to hide behind that.
"However we have made huge strides in the last six months".
Frances Crook, chief executive of the Howard League for Penal Reform, said: "It is exceptionally disturbing to see a prison with a previously good reputation collapse to such lows.
"The damning report into this overcrowded and dangerous prison is yet another symptom of our overstretched and wasteful justice system."
Nick Hardwick, HM Chief Inspector of Prisons, said: "The new leadership was aware of many of the problems and we saw early signs of determined efforts to tackle them."