Almost two-thirds of prisons in England and Wales are overcrowded with the most seriously affected having nearly twice as many men as they were built to hold.
Research by the Prison Reform Trust found there are 7,294 more people in the system than it was set up to hold, despite a recent slowing in prison population growth.
The Trust said that in July, 77 out of 131 establishments were breaching the Prison Service's Certified Normal Accommodation level, which is set by the Government to provide a "decent standard" for offenders.
The most overcrowded prison is HMP Kennet in Liverpool. Originally designed to hold 175 men, the category C prison opened in 2007 is now home to 337 prisoners.
Juliet Lyon, director of the Trust, called for an end to "short, ineffective spells" behind bars for offenders.
A Prison Service spokeswoman said: "We are aiming to reduce… crowding alongside reducing the cost of the prison estate."
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