Prison overcrowding is undermining the rehabilitation of prisoners and risks increasing reoffending in the future, a report said today.
The Criminal Justice Alliance (CJA), which represents more than 60 organisations, called for the government to urgently limit "the unnecessary use of prison, ensuring it is reserved for serious, persistent and violent offenders for whom no alternative sanction is appropriate".
It comes after Chief Inspector of Prisons Nick Hardwick said the rising pressure on prisons from budget cuts and increasing numbers cannot go on indefinitely.
The prison population in England and Wales hit 87,787 on Friday, less than 400 from the all-time high of 88,179 set last December and 98% of the usable operational capacity.
"When prisons are overcrowded, the risk that offenders will commit crimes upon release may even be greater," the report said.
A combination of "strain on prison staff, reduced access to educational and training programmes, and lack of mental health and substance abuse treatment services ... reduce the likelihood that prison sentences will actually work to tackle the causes of offending behaviour", it said.
Vicki Helyar-Cardwell, the group's director, added: "Overcrowding has, all too often, become an accepted part of life in prison, but while the system is just about coping, it struggles to meet the challenges of unexpected surge, such as those that followed the riots last year.
Judges handed out sentences almost four times as long for those convicted over the riots compared with those guilty of of similar offences in 2010, Ministry of Justice figures showed.
Helyar-Cardwell went on: "Our members report that overcrowding extracts a heavy price from prisoners, prison staff and voluntary sector working to cut reoffending, and ultimately harms communities to which ex-prisoners will return."
She added: "The last two decades show we cannot build our way out of this mess.
"Instead more should be done to divert minor and non-violent offenders out of prison into measures which enable them to make amends for their wrongdoing and better address the problems which lie behind their offending."
Lord Ramsbotham, the former chief inspector of prisons, added: "Prisons have the same role in the criminal justice system as hospitals in the NHS - they are the acute part, to which people should not be sent unless they need the treatment that only they can provide.
"There would be a public outcry if hospitals were so overcrowded that they could not provide treatment.
"There should be a similar outcry once those who read this excellent report, realise that the current level of overcrowding in our prisons impacts on public protection, because the resulting lack of treatment inevitably means higher re-offending rates."
A Ministry of Justice spokeswoman said: "Prison is necessary for the most serious and persistent offenders.
"Our prisons are there to protect the public and to punish and rehabilitate criminals so that they are less likely to return to prison.
"Our reforms aim to make both prison and community sentences more demanding, ensure victims and communities receive improved reparation and help offenders get off drugs and alcohol and into work."
She went on: "There are sufficient places for those being remanded and sentenced to custody and we are keeping the prison population under careful review.
"In the next couple of months, two new prisons are due to open which will increase the overall capacity by around 2,500 places."
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