Job losses at a male prison in London are having "serious consequences", with wardens losing track of inmates' whereabouts, inspectors have said.
HMP Pentonville also had high levels of drug use among inmates, according to a report by HM Inspectorate of Prisons.
Inspectors found that more than 1,200 men were being held at the prison, 35% above its normal accommodation level.
The chief inspector of prisons, Nick Hardwick, has questioned the prison's viability if issues are not resolved.
Inspectors said: "The staffing reductions the prison was required to make were having a number of serious consequences.
"Prisoners struggled with basic needs such as access to showers, telephones and cleaning materials."
Inspectors found the prison population to be especially "needy and challenging". Half of the inmates were being held on remand for less than six months.
The report, which was based on an unannounced inspection last summer, also found that:
- A third of prisoners told inspectors it was "easy to get drugs"
- Almost half of prisoners said they had felt unsafe in the prison at some time and almost a quarter said they felt unsafe at the time of the inspection
- On average, 19 prisoners self-harmed each month and there were about 60 prisoners on suicide and self-harm management procedures at any one time
- While the prison was combating drug and alcohol supply, positive drug testing results were high
- Prisoner movements were disorganised and "staff lost track of individual prisoners' whereabouts"
- One in 10 inmates had been assessed as malnourished when they were admitted to the prison, while about 200 prisoners were receiving opiate substitution treatment, such as methadone
- Sickness and absence were high and as a consequence the prison was operating at well below its agreed staffing levels
- The prison had "significant, easily visible vermin infestations" with many men sharing "small, dirty, badly ventilated, single cells" with broken furniture and broken windows.
Mr Hardwick said: "It is clear that Pentonville cannot operate as a modern 21st Century prison without investment in its physical condition, adequate staffing levels to manage its complex population and effective support from the centre.
"If these things cannot be provided, considerations should be given to whether HMP Pentonville has a viable future."
Frances Crook, chief executive of the Howard League for Penal Reform, said the Ministry of Justice was pushing through "unsustainable budget cuts" without addressing the overall demand prisons like Pentonville face.
But Michael Spurr, chief executive of the National Offender Management Service, the executive agency of the Ministry of Justice which runs prisons and probation services in England and Wales, said the prison would receive the support it requires.
"At the time of the inspection the prison was transitioning to new staffing profiles and new working arrangements which will provide a decent, consistent and stable regime for prisoners going forward," he said.