Prisoners at HMP Whatton are squashed in 'tiny cells' with 'leaking' water pipes, a report has found.
Major concerns have been voiced by an independent watchdog over the state of cells in B wing.
It is now calling for action to improve the facility, with the report declaring the block 'unfit for purpose.'
The Independent Monitoring Board carried out the assessment of the category C prison for sex offenders over a one year period, up to May 2019.
It stated: "The cells are tiny and well below the standard expected of modern prisons. The water pipework is deteriorating and regularly leaks. "
The board said that despite previous calls for work to be carried out by the government, "to date nothing has been done to rectify the situation and this is now an area of great concern."
HMP Whatton, near Bingham, houses around 830 male prisoners convicted of sexual offences against adults and children.
The board said the number of older inmates held at the prison continues to be a challenge.
In May 2019, 14 percent of the prison’s population were aged over 65, with some suffering with dementia, diabetes, lung disease and cancer.
Colin Braziel, chair of the Independent Monitoring Board, said: "The health care facilities at Whatton remain unfit for purpose and the board has, once again, asked the prison service to give urgent and serious consideration to their overhaul or replacement."
The report also said that 'bed watches' for ill prisoners, which meant that prison officers needed to be out of the prison and at the hospital, sometimes led to the closure of the library or gymnasiums.
It also commended the prison's education programmes as "excellent", with prisoners saying "how much lessons and teaching have helped them rehabilitate."
There were 16 assaults on staff and 45 assaults on prisoners during the reporting year, one of which was classified as serious.
There has been a total of 284 self-harm incidents, which is an increase from 223 in the previous reporting year.
Drug testing is routinely carried out in the prison, along with cell searches and the use of drug detection dogs. A mail scanner is used to scan packages for drugs.
The report states: "Despite the use of the Rapiscan (scanner) for mail and searching in visits, the availability of psychoactive substances on the wings, though limited, remains a concern."
There have been six deaths in custody during the reporting period.
Four inquests have been held, and the cause of death in all of these has been recorded as 'natural causes.'
Two inquests are still waiting to be held, the report added.
The biggest concern remains "the substandard" accommodation in B wings.
Mr Braziel added: "The board continues to highlight the poor condition of the B Wing accommodation which they regard as unfit-for purpose.
"Despite assurances from the Minister that some remedial action would be taken, there has not been any significant progress to date.
"In addition, there remains a shortage of category D places within the prison estate that will accept prisoners convicted of sexual offences in order to prepare them for release into the community."
