It is "unacceptable" that children suffering a mental health crisis are being locked up in police cells because of a lack of mental health services, the Deputy Prime Minister has said.
Nick Clegg condemned the practice of confining mentally ill people, including children, to a police custody suite when appropriate services are not available.
His comments come as a number of national organisations have signed a pledge to improve the care of mentally ill people in a crisis.
The agreement between police, mental health trusts and paramedics among others, states that police custody suites should not be used because mental health services are not available.
The Police and Crime Commissioners (PCC) organisation said that 36% of mentally ill people found in a public place who are detained by police end up in cells instead of with mental health workers.
The pledge, called the Crisis Care Concordat, challenges local services to ensure that services are always available for patients who need them urgently.
Health officials hope that the agreement will half the number of people inappropriately detained in police cells by next year. In 2011/12, 8,667 people were detained in England, the Department of Health said.
"A mental health crisis can already be distressing for individuals and all those involved, but when people aren't getting the right support or care it can have very serious consequences," said Mr Clegg.
" It's unacceptable that there are incidents where young people and even children can end up in a police cell because the right mental health service isn't available to them.
"That's why we're taking action across the country and across organisations to make sure those with mental health problems are receiving the emergency care they need. We want to build a fairer society - one where mental health is as important as physical health - and the Crisis Care Concordat is an important step towards addressing this disparity."
Care and Support Minister Norman Lamb added: " When someone has a mental health crisis, it is distressing and frightening for them as well as the people around them. Urgent and compassionate care in a safe place is essential - a police cell should never need to be used because mental health services are not available.
"The NHS and police already work well together in some areas, but it is totally unacceptable that crisis mental health care is so variable across the country. It is imperative that all areas seek to implement the principles of the concordat as quickly as possible to ensure consistent care, no matter where you live.
"Better care for people in mental health crises will not only help those living through their darkest hours to recover - it can also save lives."
The pledge also says that police vehicles should not be used to transfer patients between hospitals.
And time-scales should be developed so officers attending a crisis should know exactly how long they are to wait for a response from health and care officials.
Paul Farmer, chief executive of mental health charity Mind, said: "This is the 999 plan for mental health. It should mean that anyone in mental health crisis gets urgent and appropriate help. It is founded on the fundamental principle that mental health is not the sole responsibility of the NHS - it is everyone's business and people in crisis will only ever get the support they need and deserve if all national and local departments and services work together properly.
"Thousands of people access crisis care services every year, and countless more are turned away when they need help the most. Mind's crisis care campaign has highlighted that excellent services do exist but that they are far too patchy. Improvements to access and quality are long-overdue and it is crucial now that local services involved in the care of people in crisis pick up the baton and make this important agreement a reality in their own area."
The Royal College of Nursing welcomed the pledge as it criticised the current provision of out-of-hours care for mentally ill people in urgent need.
Dr Peter Carter, general secretary of the body, said: "Out-of-hours mental health services are woefully lacking and improved provision is urgently needed. The concordat sets out a vision for bringing about positive changes."
Mental health charity Sane questioned how mental health organisations would be able to provide the service.
Marjorie Wallace, the charity's chief executive, said: " We question how mental health trusts will be able to turn around a situation in which on some days no psychiatric beds are available in England, people are being driven hundreds of miles away from their homes to receive treatment, and last year nearly eight thousand people were taken to police cells as the only place of safety.
"We need to hear how trusts are going to be funded to enable all of them to transform the current acute shortage of beds and crisis care to meet these new, urgent requirements."
The agreement also states that there should be a 24-hour helpline for people with mental health problems.
Ms Wallace added: " Many hundreds of our callers tell us that even now, they have no telephone number to ring in times of crisis and that when they feel suicidal or have harmed themselves, they are turned away when seeking help."