Half the people dying in police custody are mentally ill, says the Independent Police Complaints Commission (IPCC).
Police watchdog calls on the NHS for emergency facilities
Officers are being left to "pick up the pieces" because of failings in health and care services, it says.
Police cells should not be used as a "safe place" for those in extreme mental distress, but all too often are the only places available, it said.
In the year to March 2004, there were 38 deaths in police custody involving people with mental illness, it added.
The figures include suicides or deaths from self-inflicted injuries, drug or alcohol related deaths and those who died after being restrained.
The IPCC wants the NHS to provide more emergency facilities to treat patients with acute symptoms.
IPCC chairman Nick Hardwick said: "At the moment police custody is regarded as a place of safety, but we do not believe that should be the case.
"The people there do not have the training or skills to deal with the issues they are confronted with."
Jan Berry, chairman of the Police Federation, said: "I don't think police officers receive the proper training and I don't think the custody suite is the most appropriate place for people with mental illnesses.
"The reality is though that there isn't anywhere else for them to go."
Representatives from the police, mental health services and voluntary groups are discussing how to make improvements to the system.
Some want to see the US Memphis scheme, where selected officers are trained by specialists to support and advise colleagues dealing with distressed mentally-ill people.
Sophie Corlett, policy director of the mental health charity Mind, argued that "police cells are for people who have committed crimes, they are never appropriate for people who are simply ill".
Places of safety
A police station is among "places of safety" defined under the Mental Health Act 1983 to which someone suffering a mental disorder can be moved.
But Paul Corry, from campaign group Rethink, said: "It is right to point to the need for the NHS to take responsibility and provide suitable crisis facilities for people, rather than leave a largely untrained police force to cope inadequately and then be blamed when tragedies occur."
A Department of Health spokeswoman said: "The Mental Health Act Code of Practice makes clear that as a general rule it is preferable for a person thought to be suffering from mental disorder to be detained in a hospital rather than a police station."