The research commissioned by the Prison Reform Trust found that the number of prisoners now serving their sentences is 7,294 more than the capacity of the prison system.
This is while that prison population growth in the country is now experiencing a modest downfall, according to the study.
The Prison Service’s Certified Normal Accommodation level set up by the government to provide a “decent standard” for offenders was being violated in July by 77 out of 131 prison buildings, the Trust said.
The Trust referred to HMP Kennet in Liverpool, a category C prison opened in 2007, as the most overcrowded prison in the country. It said the prison with an initial capacity of 175 prisoners, now holds 337 offenders.
Juliet Lyon, director of the Trust, called for “an end to short, ineffective spells behind bars for offenders”.
“We are aiming to reduce… crowding alongside reducing the cost of the prison estate”, said a Prison Service spokeswoman.