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.
The report also cites the Soham murders as it raises serious concerns over how far police still lag behind other organisations in their use of technology.
It says: "We saw the consequences of failing to exchange intelligence all too clearly in 2002 in Soham, when Holly Wells and Jessica Chapman were murdered by Ian Huntley.
"Failures to make reliable and timely intelligence available across force boundaries meant that opportunities to prevent these murders were missed."
Sir Thomas called for a service-wide mechanism to maximise effective use of technology and establish common standards.
His report says: "Until we have dissolved to nothing the remaining technological and human barriers that prevent law enforcement agencies from obtaining and using the information that others of them hold, lives could yet be shattered or even lost."
The wide-ranging assessment of the state of policing in England and Wales in 2016 also:
:: Notes the changing nature of crime including the "modern tsunami" of online fraud;
:: Highlights that 18 out of 43 forces require improvement in at least one of the watchdog's main inspection themes;
:: Warns that children are now exposed to "very great dangers" which are often "greater and more prevalent in the online world than they are in the physical world";
:: Says neighbourhood policing continues to be eroded;
:: Raises concerns that not all forces accurately record all reported allegations of rape.
Sir Thomas paid tribute to the bravery of officers.
"Every day and every night, police officers do things that most of us would go out of our way to avoid," he said.
"This has been illustrated to a tragically graphic extent by the Westminster terrorist attack in which one very brave police officer, Pc Keith Palmer, lost his life."
Sara Thornton, chairwoman of the National Police Chiefs' Council, said: "Being mentally unwell is not a crime and those experiencing a health crisis are not criminals.
"Over the last two years, forces have reduced the use of cells as a place of safety by nearly 70%, and by over 80% for children.
"However, the best response - both to crime and mental health - will never be better than preventing harm before it happens. All agencies must work together to put appropriate resources in place, share learning and intervene early. Getting this right can be not just life-changing but life-saving."
She added that forces are "continuing to perform well overall" and the majority were graded good or outstanding across all three areas of inspection.