Thousands of people with mental health problems are being detained in police cells rather than being taken to hospital for assessment, a report says.
Over a year, more than 11,500 people were held in police custody for assessment under the Mental Health Act - double the number taken to hospital.
The Independent Police Complaints Commission collated data for all 43 police forces in England and Wales.
IPCC commissioner Ian Bynoe said the situation was "intolerable".
Section 136 of the Mental Health Act enables officers to take someone they believe is suffering from a mental disorder, and in need of immediate care or control, from a public place to a "place of safety" where they can be examined by a doctor and interviewed by an approved social worker.
The continued use of cells not only diverts police resources from fighting crime, but criminalises behaviour which is not a crime
IPCC commissioner Ian Bynoe
It is generally agreed that police custody should be used only in exceptional cases.
But the IPCC report suggests it is being used as the main place of safety.
Wide variations occurred across police forces in the use of police custody as a place of safety and this was largely explained by the availability of alternative places of safety.
Cheshire Police and Merseyside Police reported low levels of detention under section 136 (one per 10,000 people in custody), while Sussex Police and Devon and Cornwall Police reported high rates (277 per 10,000 and 174 per 10,000 respectively).
The IPCC found examples where forces had received funding to build new dedicated places of safety, but no money for staffing, and so they remained unused.
Police feel under-resourced and under trained to deal with mental health issues
A spokeswoman for the mental health charity Mind
Mr Bynoe said: "Police custody is an unsuitable environment for someone with mental illness and may make their condition worse.
"The continued use of cells not only diverts police resources from fighting crime, but criminalises behaviour which is not a crime."
Dr Michele Hampson of the Royal College of Psychiatrists said: "Anecdotally, we have heard of new units which are unable to open for lack of staff, and others that expect the police to remain until the assessment has been completed.
"Sufficient staff must be available at short notice to enable these facilities to cope with all but the most disturbed individuals or those needing medical care. This must be given the priority it deserves."
A spokeswoman for the mental health charity Mind said: "We've been told anecdotally that police feel under-resourced and under-trained to deal with mental health issues, and the report highlights the need for mental health awareness training."
Phil Gormley, deputy chief constable of West Midlands Police and Acpo lead on mental health and disability, said: "Acpo is already developing further guidance and training to inform the police service response to people with mental health issues and we are involving and consulting the IPCC throughout this process."
Louis Appleby, the government's national director for mental health services in England, said the government had made clear that police stations should only be used as a place of safety in exceptional cases.
Mr Appleby said that, since the IPCC survey was carried out, significant funding had been made available.
He said: "Primary care trusts should make sure they have local policies in place, jointly agreed with the police, to make sure that people with mental health problems are treated in appropriate settings."
The government said that, from April 2006, the Department of Health had provided Ŗ130m available to the NHS in England for investment in adult mental health services.
The Royal College of Psychiatrists has established a multi-agency group to develop a new set of standards on the use of Section 136, which are due to be published on 29 September 2008.
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