Inmates are forced to "double up" in single cells due to overcrowding
Two prisons in the South East have been criticised in a study published on Wednesday.
But two other jails have been praised for their safety in the survey which was carried out by the Prison Reform Trust.
The report found Canterbury Prison had the fourth highest rate of overcrowding in the country.
The Prison Service's target is to have only 18% of inmates doubled up in cells designed for one person.
But at Canterbury this figure was found to be 80.5% compared to the national average of 20.4 %.
Lewes Prison in East Sussex was found to have a high suicide rate with three inmates taking their own lives.
However it was not as bad as Durham Prison, where six people killed themselves, or five other jails where four prisoners took their own lives.
Praise was given to two prisons in Kent with Blantyre House, near Goudhurst, and East Sutton Park, near Maidstone, named as two of the safest jails in the country.
Both prisons had a 0% overcrowding rate and a 0% record of assaults.
Reverend Robin Carter, governor of HM Prison East Sutton Park, said: "Prisons are all individual - they are so different.
"I do not have many of the problems that my colleagues at more secure and closed prisons have.
"But none the less I have a range of women here who are serving anything from life to a few months who come to us with a whole range of needs but it is how we deal them.
"It is dealing with people humanely and being caring."
The Prison Reform Trust study is based on the Prison Service's own data for England and Wales, published last month.
The trust claims that overcrowded jails are leading to poor performances in other areas as the system struggles to cope.
Enver Solomon, author of the report, said: "This report reveals a Prison Service whose performance is being severely hampered by a record prison population.
"Far too many prisons are overcrowded, provide inadequate levels of
purposeful activity and experience high levels of recorded drug use.
"The only way to improve these jails is immediately to reduce their