Too many people suffering from mental disorders are being locked up in police custody rather than being protected in hospitals, a joint inspection has found.
Police have powers to take individuals suffering from mental health issues in a public place to a "place of safety" for their protection - but guidance states that in all but "exceptional" circumstances this should be in a hospital or health location.
More than 9,000 people in 2011/12 were taken into police custody using powers under section 136 of the Mental Health Act, a joint report by HM Inspectorates of Constabulary and Prisons, the Care Quality Commission, and the Healthcare Inspectorate Wales said.
This is "clearly not an exceptional use of the power", the inspectors added.
Some of the most common reasons given for holding these individuals in police custody were insufficient staff and the absence of available beds at a health-based place of safety.
Speaking on behalf of all inspectors, Her Majesty's Inspector of Constabulary Drusilla Sharpling said: "This report finds that too many people are being detained in police custody under section 136.
"Their only 'crime' is that they have mental disorders, but they are treated in many ways as if they are criminals. This deplorable situation cannot be allowed to continue."
In the vast majority - 81% - of the 70 cases examined as part of the inspection, the reason for detention was that the person had either attempted suicide or self-harm, or indicated that they were thinking of doing so.
People detained in police custody under section 136 are subject to the same processes and procedures - and kept in the same style of cell - as those arrested for crimes, the inspectors said.
Police custody cells are not designed to support the needs of detainees with mental health disorders but the inspection found these detainees spend an average of 10 hours and 32 minutes in custody.
The law allows them to be detained for up to 72 hours, without review, while those arrested for a crime can only be held for a maximum of 24 hours.
Looking after mental disorder detainees until an assessment can be made or a more appropriate place of safety found also means that police officers are taken off their more regular duties, the inspectors added.
On one occasion, a custody sergeant was so concerned about the welfare of a 17-year-old detained in police custody under section 136 overnight that he placed him on "close proximity observations", which requires either a police officer or police staff member to monitor the detainee constantly from outside the cell.
In many of the 70 cases examined in detail, the reason why the police used custody as a place of safety, rather than a hospital, was not given.
But when it was recorded, the most common explanations were insufficient staff at a health-based place of safety and the absence of available beds.
Police officers told inspectors that they do not use section 136 lightly and many did not believe police custody was the appropriate place for these people to be.
The report calls for commissioners of health, mental health and social services to ensure that they put the resources in place to receive people detained under section 136 in a health-based setting.
Police and crime commissioners (PCCs) welcomed the inspectors' report
The Association of PCCs, which represents all 41 of the commissioners in England and Wales, said: "People with mental health issues require the best possible support and care.
"It is a police officer's role to ensure the safety of the public and individuals who pose a threat to themselves or others.
"Too often police officers and police staff are dealing with people with complex mental health needs rather than with the support of experienced and trained medical professionals.
"PCCs support the HMIC call that resources are put in place by commissioners of health, mental health and social services to ensure that people detained under section 136 are assessed quickly in a health-based setting."
A Home Office spokesman: "We agree that it is unacceptable for people with mental health problems to be kept in police cells.
"As the home secretary said in her speech to the Police Federation, we are already working on a range of measures to ensure that people with mental health problems get the care they need.
"These include providing suitable places of safety in every local area and piloting street triage services where mental health nurses accompany police officers to incidents."