There has been a steep rise in the number of people arriving at A&E departments in England with mental health problems, figures show.
Experts say a lack of early support means patients are reaching crisis.
Data compiled for the BBC by NHS Digital showed that between 2011-12 and 2015-16 the number of patients attending A&E units with psychiatric problems rose by nearly 50% to 165,000.
For the under 18s alone the numbers almost doubled to nearly 22,000.
These figures represent a small minority of overall A&E attendances - just over 1% in total.
But the Royal College of Emergency Medicine said that was likely to be the "tip of the iceberg" as these figures just included cases where the primary diagnosis was a psychiatric condition.
Patients coming in with self-harm or after an attempted suicide may have been recorded as having a different reason for attending hospital.
Some trusts the BBC spoke to said as many as a tenth of patients were attending A&E because of mental health problems.
Analysis: By Alison Holt, BBC social affairs correspondent
A 999 call comes in. The man says he is ill, he's hearing voices and wants to kill someone.
Usually emergency services would be despatched and he'd probably be taken to an already overstretched accident and emergency department. But in Birmingham, the street triage team is sent with a paramedic, police officer and psychiatric nurse on board.
By intervening at this stage, they aim to get the patient appropriate help and ease pressures on A&E - the man has been taken there about half a dozen times already in the last few weeks.
After talking to the team and physical health checks, it's agreed he'll remain at home and keep an appointment in the morning.
The initiative set up by the Birmingham and Solihull Mental Health Trust has reduced the numbers in mental health crisis going to A&Es locally. As part of RAID - Rapid Assessment, Interface and Discharge - psychiatric teams are also based in emergency departments. They aim to see people with psychiatric problems within an hour.
Dr Peter Aitken, of the Royal College of Psychiatrists, said the figures were "extremely concerning".
"Years of underfunding has meant these children have not been able to benefit from early intervention."
He said A&Es needed to invest in specialist psychiatric services to ensure people with mental health problems got the right support.
"Mental health presentations to A&E are unlikely to be minor as they represent an urgent crisis point in a person's life," he added.
Dr Marc Bush, chief policy adviser at the charity Young Minds, called for more investment. "Far too many young people end up in A&E, which can be crowded and stressful, because there's nowhere more appropriate for them to go," he said.
"One of the main reasons that crisis services are so overstretched is that young people who are struggling don't get help soon enough, which means that problems often escalate."
Marjorie Wallace, chief executive of mental health charity Sane, said: "The reported steep rise in the number of patients attending A&E units with psychiatric problems demonstrates the stark reality that many people in mental distress, some suicidal, feel they have nowhere else to turn.
"Unless resources are put urgently into community and in-patient services, people in mental health crisis will be failed, sometimes with tragic consequences."
The figures have been revealed just a day after the prime minister announced a package of measures to support people with mental health problems.
A National Society for the Prevention of Cruelty to Children spokesman added: "These figures are deeply troubling and highlight the urgent need for frontline mental health support for vulnerable children.
"We know that up to nine in 10 children who are abused will develop a mental illness by the time they're 18, and many aren't getting the therapeutic support they desperately need to rebuild their lives - too often it comes only when a child is on the brink of suicide."
A spokeswoman for the Department of Health said: "As the prime minister has made clear this week - improvements must be made in mental health care both within and outside of the NHS, to make sure people get the support they need before they reach crisis point."
She said extra money being invested in the NHS this Parliament would help ensure improvements take place.