Many prisoners have mental health problems
Thousands of people with mental health problems are ending up in jail rather than receiving treatment, the Prison Reform Trust has said.
Offering mental health and social care instead of custody would relieve pressure on prisons and could cut reoffending rates, the trust argues.
It says figures show one in 10 inmates has a "serious mental health issue".
The government said it had made it clear that offenders with severe mental illness should be treated not punished.
The trust report adds that 90% of inmates have at least one diagnosed mental health disorder.
Its report is based on information from 57 Independent Monitoring Boards - citizen groups which regularly visit prisons in England and Wales.
It paints a bleak picture of life in prison for those with serious mental illnesses and cites the following as typical examples of poor treatment:
• A prisoner who made several cuts to both arms with a razor blade was treated by a nurse and put into the healthcare unit. There was no other treatment available to aid recovery.
• A young female prisoner with a mental age of a young adolescent was put in a segregation unit for 28 weeks because staff wanted a respite from her behaviour.
• A prisoner who had a clinically assessed mental age of between two and five was cared for in the care and separation unit. His toileting and washing needs were met by nurses until he was transferred out of the prison.
Juliet Lyon, director of the Prison Reform Trust, said: "This unique report raises searing questions about why we lock up our most ill people in our most bleak institutions.
"Why waste time and public money building bigger and bigger prisons when it is clear that our jails are full of people in urgent need of proper mental health and social care?"
The trust's head of policy, Imran Hussain, said prisons were being used as a "social dustbin".
"Everyone accepts that there are plenty of people in prison who should not be there," he said, adding that it was more cost-effective to treat than to incarcerate.
Peter Selby, president of the National Council for Independent Monitoring Boards, said there was "no more distressing mismatch in our justice system than mental illness and prison".
He said that too often prison officials were doing work better carried out by skilled health professionals.
The trust wants to see a national network of specialists created who could divert offenders into mental health care treatment if safety considerations allowed.
In addition, every prison should have a learning disability specialist and care should continue once a prisoner had left jail, it argues.
The release of the trust's report comes as a government review of mental health care in prisons is due to be submitted to ministers.
A Ministry of Justice spokesperson said: "The government is clear that, wherever possible, mentally disordered people who offend should be treated rather than punished and the courts can order the admission of any defendant to hospital rather than prison.
"It is Prison Service policy that prisoners with severe mental illness are transferred to receive the appropriate treatment in hospital.
"And the process by which prisoners are transferred to appropriate hospital accommodation has improved."