Northumberland Prison is running out of cells, with campaigners warning that the safety of staff and inmates is at risk as the prison population increases.
The latest Ministry of Justice figures show that the prison has capacity for 1,348 inmates.
In July, it was operating at 99% of its capacity, with room for just 11 more prisoners.
Campaigners say that the unchecked rise of the prison population is responsible for the huge increase in assaults on staff and other inmates, culminating in the Government taking over HMP Birmingham from its contractor, G4S, after a damning inspection report.
One of the Government’s first actions was to move hundreds of inmates to other jails, reducing the overcrowding.
The Prison Service measures its own capacity in terms of Certified Normal Accommodation – the number of prisoners it says it can accommodate in the “good, decent standard of accommodation that the service aspires to provide all prisoners”.
However, with the majority of prisons overcrowded across England and Wales, it also has a separate measure called Operational Capacity. This is the maximum number of prisoners the Prison Service says each institution can safely handle while maintaining control and security.
In some prisons, such as Northumberland, inmates cannot be safely housed together in cells, so the Certified Normal Accommodation and the Operational Capacity are the same.
Prisons contain a number of one and two-person cells. In overcrowded prisons, more inmates will be put in cells than they were originally designed to hold.
Annual figures, published by the Prison Service in July, show the extent of the problem. From April 2017 to March 2018, 1% of prisoners in Northumberland Prison were in overcrowded cells, 16 inmates on average.
Prison Reform Trust director, Peter Dawson, said: “Overcrowding isn’t simply a case of being forced to share a confined space for up to 23 hours a day where you must eat, sleep and go to the toilet.
“It directly undermines all the basics of a decent prison system, including work, safety and rehabilitation.
“No government has succeeded in building its way out of overcrowding. So we need a fundamental rethink about who we send to prison and for how long.”
There has been a renewed focus on prison policy after the Government took control of HMP Birmingham.
A shocking inspection found inmates used alcohol, drugs and violence with impunity, while the corridors were covered with cockroaches, blood and vomit.
Chief inspector of prisons, Peter Clarke, said it was “some of the most disturbing evidence” inspectors have seen in any prison.
Across the country, assaults have more than doubled in prisons over the last five years, and cases of self-harm have increased by 93%.
Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said: “The fact that Birmingham prison is seeing its population reduced by 300 to help enable the government to improve conditions there is a tacit admission that prison overcrowding plays a key part in why our prison system fails more generally.
“Cramming more people into prisons than they were designed to hold is a recipe for violence, drug abuse and mental distress.
“Bold action is needed to reduce the number of people behind bars and ease the pressure on other prisons.”
A Prison Service spokesman said: “All prisons in England and Wales are within their operational capacity which means they are safe for inmates.
“Nonetheless, reducing crowding is a central aim of our modernisation of the prison estate.
“That is why we have committed to delivering up to 10,000 new prison places across the country, and only last month announced that two new prisons would be built at Wellingborough and Glen Parva.”