Too many people with mental health problems are ending up in prison when what they need is specialist care, a Prisons Inspectorate report suggests.
Prison overcrowding has become a matter of growing concern
Despite the ongoing concern about overcrowding, prison had become the "default" option for many who posed little public risk, it said.
Once inside, inadequate screening on arrival coupled with poor communication between agencies made matters worse.
The report argued that to tackle the problem, a true alternative was vital.
Many prisoners with mental health disorders did need to be in prison, said Chief Inspector of Prisons Anne Owers, but institutions were currently unable to provide them with the care they needed.
The professional healthcare they required was simply not forthcoming, while overcrowding meant that support from staff was often unavailable.
The report added that resources should not be used up on people who should not be in prison in the first place.
Boosting care services
Better GP training
Greater range of treatments
"Prisons can provide better and more focused care for those who need to be there," said Ms Owers.
"But they will only do so effectively if there is sufficient alternative provision for those who should not be there, and effective community support for those who leave prison."
Any alternative, the report stressed, would have to ensure the public was protected at the same time as the patient's needs were met.
In a high-profile court case this week, a paranoid schizophrenic was detained indefinitely after stabbing to death a mental health charity worker.
He had been receiving voluntary psychiatric treatment since being convicted of causing grievous bodily harm after attacking his parents with a hammer in 1994.
The inspectorate was unable to provide figures on the number of people it thought should not be in prison, but suggested that better mental health provision could help lower soaring prison populations.
"There are people who, if they were picked up earlier, need not get so risky as they became, and there are people who are in prison with very low risk simply because there's no community provision," said Ms Owers.
Responding to the report, Sean Duggan of the Sainsbury Centre for Mental Health said current staffing levels in prison mental health care were "only one third of what is needed".
"We urgently need more investment, especially in primary care within prisons, to close that massive gap."
A Ministry of Justice spokesman said the government welcomed the report.
He added: "We are working to improve the areas identified in the report - for example reception screening is currently being reviewed, and we are working to produce guidance this year which will improve the operation of court diversion schemes for mentally ill offenders."
Steve Shrubb, the director of the NHS Confederation Mental Health Network, said: "The answer to this can not be simply the commissioning of more services. As the report itself suggests the key is to involve the whole range of available services and groups ¿ from commissioners right down to the families of prisoners ¿ working together."