Two Doncaster prisons are forcing prisoners to share cells, eating, sleeping and even using the toilet in spaces designed for one.
And another is running out of cells.
Ministry of Justice figures show overcrowding in Doncaster and Lindholme and Hatfield Prisons.
They reveal that 1,087 prisoners ewre fitted into 738 spaces at Doncaster Prison in July.
Campaigners say the rise of the prison population is responsible for rising assaults on staff and other inmates.
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 - the maximum number of prisoners the Prison Service says each institution can safely handle while maintaining control and security.
In July, Doncaster stood at 95 per cent of the Operational Capacity.
Prisons contain a number of one and two-person cells. In overcrowded prisons, more inmates are put in cells than they were originally designed to hold.
From April 2017 to March 2018, 65 per cent of prisoners in Doncaster Prison were in overcrowded cells, 734 inmates on average.
The figures also show 925 prisoners at Lindholme put into 884 spaces at the prison in July. It also stood at at 95 per cent of the Operational Capacity.
From April 2017 to March 2018, 17 per cent of prisoners in Lindholme Prison were in overcrowded cells, 165 inmates on average.
The figures show Hatfield has capacity for 378 inmates. In July, it was operating at 99 per cent of its capacity, with room for just five more prisoners.
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."
Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said: ““Cramming more people into prisons than they were designed to hold is a recipe for violence, drug abuse and mental distress."
The Prison Service says all prisons in England and Wales are within their operational capacity which means they are safe for inmates but reducing crowding is a central aim of the modernisation of the prison estate.
It plans to create up to 10,000 new prison places across the country.
Serco, which has an agreed operational capacity with the Ministry of Justice to deliver 1,145 spaces at HMP Doncaster, says the figure has been risk assessed against the current staffing levels and is sufficient to deliver a safe and decent regime.
Doncaster is rated 3/4 iin the latest MoJ reports, meaningit meets the majority of targets. Birmingham was rated as 1/4, meaning 'performance is of serious concern.