Police have been left to pick up the pieces amid a national crisis in mental health care, a watchdog report has warned.
Stretched forces are responding to tens of thousands of cases that would be better dealt with by other agencies, HM Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS) said.
Officers are often required to step in after more suitable services have gone off duty, the assessment found.
It noted that the "intolerable burden" comes at a time when police are facing demands in other areas including a heightened terror threat and mounting levels of knife crime.
HM Inspector of Constabulary Zoe Billingham (pictured) said the inspection found police respond to people with mental health problems with care and compassion.
"But we cannot expect the police to pick up the pieces of a broken mental health system," she said.
"Over-stretched and all-too-often overwhelmed police officers can't always respond appropriately, and people in mental health crisis don't always get the help they need.
"It is a national crisis which should not be allowed to continue."
Ms Billingham said other services "need to stop relying on the 24/7 availability of the police".
She added: "The police should be the last resort, not the first port of call.
"We are seeing forces attending less other crimes because they are focusing on mental health-related incidents."
HMICFRS assessed the response provided by forces in England and Wales to people with mental health problems.
Police involvement could include responding to emergency contacts from concerned relatives or friends, making welfare checks at the request of social services or GPs, supporting victims of crime who have mental health problems, attending incidents where someone is suicidal or searching for missing people.
The full extent to which people with mental health problems place demands on police is not fully understood, the watchdog found.
It said figures showing that 97,796 crimes and 431,060 incidents were flagged as involving mental health concerns in the year to June 2017 seem "exceptionally low".
The inspectorate flagged up separate statistics showing that, in London, police receive a call about a mental health concern once every four minutes and send an officer to respond to a mental health call every 12 minutes.
The top five individual repeat callers to the Metropolitan Police all have mental health problems and called the force a total of 8,655 times in 2017, the report said. It cost the force an estimated £70,000 just to answer the calls.
Half the time police rather than the ambulance service transport people who are detained under the Mental Health Act to a "place of safety", according to HMICFRS.
Its analysis found that the peak time for calls to police for support with mental health-related incidents is between 3pm and 6pm Monday to Friday.
Noting that GP surgeries, social care and community mental health teams "tend to finish work at 5pm", the report said: "Often, as a 24/7 service, police are the only professionals available to respond because the person is in crisis 'out of hours'."
Examples of police stepping in to fill shortfalls in health services could include transporting someone to hospital when an ambulance is not available or waiting at hospital until a mental health place is found.
The findings come amid intense scrutiny of police funding and performance.
Arrests in England and Wales have halved in a decade, while recorded crime has risen recently in a number of categories including homicide and knife-related offences.
Police funding has fallen by 19% in real terms since 2010, while officer numbers have dropped by more than 20,000 over the same period.
A Government spokesman said: "The NHS has worked closely with policing partners to reduce the use of police custody as a place of safety by 95% since 2011/12.
"We are investing £2 billion in mental health services, including mental health liaison in A&E departments, and community crisis services, and NHS England will shortly be setting out its proposals to improve all mental health services in the long-term plan.
"Police officers do an excellent job protecting those facing mental health problems in often difficult and distressing circumstances and it is right that this report acknowledges police leadership in this area to be strong."
Dr Paul Lelliott, lead for mental health at the Care Quality Commission, said: "People experiencing a crisis with their mental health need expert and prompt help. All too often this isn't available at the time and place that they need it.
"Although police officers generally do a good job in identifying and responding to those with mental health problems, they must never be considered a substitute for expertly trained healthcare professionals."
Chief Constable Mark Collins, the National Police Chiefs' Council lead for mental health and policing, said: "We share Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services' concerns that too many people are being directed to police where they should instead be receiving expert support from healthcare professionals.
"It is right that the police are there to protect those in immediate danger, but they shouldn't become the first point of call for those who need longer term mental health support and access to prevention measures."
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