The Trust lists England and Wales's 20 most overcrowded jails
Almost two-thirds of prisons in England and Wales are overcrowded, with jails' intended numbers exceeded by almost 9,000, the Prison Reform Trust said.
Last month, 88 of 140 jails were overcrowded, the Trust added. Worst was Shrewsbury Prison with 316 inmates, 79% more than its normal population of 177.
The Trust says the problem increases reoffending, and wants prison reserved for serious and violent criminals.
The Prison Service said jails remained "places of both punishment and reform".
"We will always provide enough prison places for serious and persistent offenders," said a service spokesman.
A total of 4,929 places had been added to the prison estate since 2007, with another 1,750 due this year, he said.
"This is intended to provide us with sufficient space to modernise the estate and ensure prisons remain places of both punishment and reform."
Ministry of Justice (MoJ) figures put the prison population at the end of last month at 83,962 - 8,865 above the normal accommodation level of 75,097.
Juliet Lyon, director of the Prison Reform Trust, said: "Pressure on public spending means that ministers can no longer afford to be complacent about prison overcrowding or the high reconviction rates it leads to.
"Simply building more prisons is an expensive dead end."
The Trust blames overcrowding in jails on factors including:
- "Sentence inflation", with more and longer jail terms handed out in recent years
- Use of indeterminate "public protection" sentences, and some inmates' lack of opportunity to demonstrate that they pose no risk to the public
- "Unnecessary" use of imprisonment for some some people awaiting trial or sentence, and some licence condition breaches
"Overcrowded jails are not working to prevent re-offending," the report adds.
It highlights 2004 Home Office figures that suggested almost 65% of prisoners were reconvicted within two years of release, with the figure over 75% for 18- to 20-year-olds.
'University of crime'
The figures bear out "what many fear - that prison acts as a university of crime for the young", the report says.
Prison staff shortages, reduced opportunities for constructive activities due to budget cutbacks, repeated transfers between prisons, and imprisonment far from home leading to reduced family visits and support, all hindered prisoners' rehabilitation, the Trust said.
Geoff Dobson, the Trust's deputy director, said: "We are kind of trying to be tough all the time, rather than effective.
"We think that more money spent on crime prevention, drug treatment and facilities for dealing with mental health problems, would be far more effective.
"Then prison could be doing a better job for the serious offenders who need to be there."
In Scotland last month, the prison population peaked at 8,584 - 1,144 above the designed capacity.
However, 370 of those inmates were on home detention, the Scottish Prison Service said.
Meanwhile, in Northern Ireland, the population peaked at 1,542 in July - 113 below normal accommodation levels.
While some types of prison, such as women's facilities, were under capacity, others were overcrowded, the Northern Ireland Prison Service said.
Andrew Neilson, deputy director of the Howard League for Penal Reform, said the prisons had been "blighted" by overcrowding since the early 1990s.
"The situation is now so bad that an early release scheme is in place just to make space," he said.
The England and Wales prison population reached a record high this month, exceeding 84,000 for the first time.
This is despite more than 60,000 inmates benefiting from an early release scheme - freeing them up to 18 days early - introduced in June 2007.
Mr Neilson said: "The only answer to this is sentencing reform and an acknowledgement that we send too many of the wrong kind of people to prison."
Home Office figures show that, 10 years ago, the average prison population was 64,771. Ten years before that, in 1989, it was 48,610.
Its definition of overcrowding is a level of inmate density beyond "the good, decent standard of accommodation that the Service aspires to provide all prisoners".