Prisons in England and Wales are "bursting at the seams" with more than half suffering overcrowding, the Howard League for Penal Reform has claimed.
Leicester Prison had 361 inmates in January
A report by the organisation says the prison population has reached an all-time high of 75,479 inmates.
Leicester Prison is the most overcrowded with 90% more inmates than it has places for, followed by Preston which has 80% and Shrewsbury at 73%.
The Home Office said projects were in place to increase prison capacities.
The study claims 76 of the 139 prisons in England and Wales were overcrowded in January.
Leicester overcrowded by 89%
Preston overcrowded by 80%
Shrewsbury overcrowded by 73%
Swansea overcrowded by 65%
Bedford overcrowded by 61%
Exeter overcrowded by 58%
Dorchester overcrowded by 57%
Leeds overcrowded by 55%
Lincoln overcrowded by 49%
Wandsworth overcrowded by 49%
Leicester Prison has 191 places but Howard League figures show it had 361 prisoners - making it the most crowded in the country, according to the group.
Howard League director Frances Crook said: "Our prison system is bursting at the seams.
"Overcrowded prisons cannot provide a suitable regime to rehabilitate prisoners, which means that you and I are not safe on the streets."
She said: "Prisoners in overcrowded prisons are more likely to share cells built for a single occupant.
"In 2003-2004, the average rate of "doubling", which means two people share a cell built and designed for one, was 21.7% - far exceeding the target."
The organisation believes only serious offenders should be locked up and has called for more use of community sentences.
A spokeswoman for the Home Office said it was down to the courts to decide when to send people to prison.
She added there were a number of projects to increase operational capacity at prisons across the country.