Many prisoners have mental health problems
More must be done to provide expert care to prisoners with severe mental illnesses, a study suggests.
Treating prisoners in secure hospitals can help save £600,000 in re-offending costs, but not enough are getting help, analysts Laing and Buisson said.
The lifetime savings includes costs associated with police time, emotional harm caused to victims and legal fees as well as the bill for the NHS care.
The government said steps were being taken to improve access to care.
It is estimated that 1.5% of the 80,000-plus prison population in England and Wales at anyone time has conditions severe enough, such as schizophrenia, that require hospital treatment.
But latest figures suggest less than half of these actually get it.
If all these prisoners were to get help, the lifetime saving would add up to £1bn.
The analysts believe this research, compiled from previously published data, presents a compelling argument to get improved access to treatment.
Report author Dr Judy Renshaw said: "Although the large majority of prisoners with mental illnesses can be treated effectively within prisons, the criminal justice system needs to recognise that there is a small minority with severe mental illness who remain in prison to the detriment of their own mental health."
Andy Bell, of the Sainsbury Centre for Mental Health, said the authors were right to highlight the issue.
"Improving access to secure hospital facilities is thus vital, but we also need to ensure we have the right mix of services that includes step-down and community services to reduce 'bed-blocking' in expensive medium secure hospitals."
The government said it was already an issue it was attempting to tackle.
A spokeswoman for the Department of Health, which has been taking a led on prisoner health care in Whitehall, said staff training was being improved and there was now a drive to give people access to the appropriate care.
"Over time these measures will help reduce use of prison for people with mental health problems as well as provide better services in prisons."