Public safety is being placed at risk because police are having to plug gaps in other public services, a watchdog has warned.
Forces are increasingly used as a service of "first resort" to help those with mental health problems, HM Chief Inspector of Constabulary Sir Thomas Winsor said.
Publishing his annual State of Policing report, Sir Thomas said: "The severe problems in mental health provision in this country are not only failing those who need treatment; they also create an unacceptable strain on the police and imperil public safety."
The assessment found that in some areas officers are acting as first responders when no ambulances are available.
Inspectors also continue to encounter cases of mentally ill people who have not committed any crime spending the night in a police cell.
Sir Thomas said the provision of mental healthcare has reached "such a state of severity" that police are often being used to fill the gaps other agencies cannot.
"This is an unacceptable drain on police resources, and it is a profoundly improper way to treat vulnerable people who need care and help," he said.
"The first obligation of the police is to prevent crime. This is not only because this makes society safer - both in reality and in perception - but also because it is far cheaper to prevent a crime than it is to investigate and arrest the offender after the event.
"The same is true of mental ill-health, which is not a crime."
Until mental health is given the same priority as physical health in resources including funding, the police will continue to play too large a role in dealing with people with mental health issues, the report warns.
It says that by the time officers become involved, many opportunities will already have been missed.