Nearly two-thirds of prisons in England and Wales are overcrowded, according to the Prison Reform Trust.
The trust said there were 7,294 more people in the system than it was designed and built to hold.
HMP Kennet in Liverpool, Merseyside, is the most overcrowded prison in England and Wales, followed by Shrewsbury and Swansea, according to the trust.
A Prison Service spokesman said it was aiming to reduce the existence of crowding and the cost of prison estate.
He added: "All of our prisons provide acceptable levels of accommodation for prisoners, although some prisons hold more people than they were originally designed for.
Although the growth in the prison population has slowed down in recent months, there were 77 out of 131 establishments operating over the Prison Service's "certified normal accommodation" (CNA) as of 27 July, the trust said.
Ten most overcrowded prisons
- Kennet, Liverpool
- Shrewsbury, Shropshire
- Swansea, South Wales
- Leicester, Leicestershire
- Exeter, Devon
- Dorchester, Dorset
- Wandsworth, London
- Northallerton, North Yorkshire
- Preston, Lancashire
- Doncaster, South Yorkshire
CNA is "the good, decent standard of accommodation that the service aspires to provide all prisoners", a trust spokesman added.
HMP Kennet in Liverpool, the most overcrowded prison in England and Wales, is designed to hold 175 men but holds 337.
In second place, the most overcrowded prison is Shrewsbury - built to hold 170 men, but holding 326; and third is Swansea which is built for 240 but holds 436.
The trust spokesman said: "For people in prison themselves, overcrowding has a tangible impact."
The trust said that figures for 2010/11 show that nearly a quarter of people in prison were being held in overcrowded accommodation, either doubling up in cells designed for one occupant or being held three to a cell in cells designed for two people.
Private prisons have held a higher percentage of their prisoners in overcrowded accommodation than public sector prisons every year for the 13 years to 2010/11, it said.
"Overcrowding makes it much harder for staff to work intensively with offenders on resettlement," said the spokesman.
"Currently 47% of adults reoffend within a year of leaving prison, rising to almost 57% for those who had served a sentence of less than 12 months."
'Curb sentence inflation'Juliet Lyon, director of the trust, said: "Building our way out of the overcrowding problem is not the answer.
"The prison population can be safely reduced by curbing inflation in sentencing, calling a halt to any unnecessary use of custodial remand, dealing with addictions and investing in effective community penalties.
"Rather than falling back on short, ineffective spells behind bars, investment in more intensive community sentences and public health solutions would cut crime and save the taxpayer money."
The prison population currently stands at 86,801, as of Friday 17 August.
The same time last year, it was 86,233, and five years ago it was 80,762.
The average population in custody during 2002 was 70,860.
