Overcrowding at Peterborough Prison is forcing prisoners to share cells, eating, sleeping and even using the toilet in spaces designed for one.
Ministry of Justice figures show that 1,190 prisoners were crammed into just 1,119 spaces at the prison in March.
Campaigners say that the unchecked rise of the prison population is responsible for the huge increase in assaults on staff and other inmates - a situation described last week as a "national emergency".
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, they also have a separate measure called Operational Capacity. It is the maximum number of prisoners the Prison Service says each institution can safely handle while maintaining control and security.
In March, Peterborough's population stood at 94% of this capacity.
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.
Figures released last month showed that 771 assaults were recorded at Peterborough in 2017, nearly triple the number in 2012. Of those attacks, 297 were on staff.
There were also 1,809 cases of self-harm recorded in the prison last year, an increase of 44% on 2012.
The director of the Prison Reform Trust, 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.
"Despite a virtually permanent programme of prison building, overcrowding has been an unchanging reality of our prison system since 1994. Building prisons isn't the solution - breaking our addiction to imprisonment is."
Across the country, assaults have more than doubled in prisons over the last five years, and cases of self-harm have increased by 93%.
Frances Crook, chief executive of the Howard League for Penal Reform, said last week: "This shameful rise in violence and self-injury is the direct result of policy decisions to allow the number of people behind bars to grow unchecked while starving prisons of resources.
"This is a national emergency, and the government must respond boldly and urgently."
A spokesman for the Ministry of Justice said: "Prison numbers can fluctuate, which is why we have robust plans in place to ensure we always have enough prison places for those sent to us by the courts.
"We will always ensure there are enough cells across the prison estate, and manage this in a way that gives taxpayers the best possible value for money.
"We are investing £1.3 billion to build modern new establishments, with up to 10,000 new prison places and better education facilities."