The proportion of children and young people who report having a mental health condition has risen six-fold in around two decades, a new study has found.
In 1995, just 0.8% of four to 24-year-olds in England said they had a long-standing mental health condition, according to the research published in journal Psychological Medicine.
This rose to 4.8%, the equivalent of almost one in 20 young people, by 2014.
There were also notable increases in Scotland and Wales.
The researchers, from University College London, Imperial College London, University of Exeter and the Nuffield Trust, described the rise as "striking".
It is believed to be the first national study in more than 10 years to investigate trends in mental health problems among young people in the UK, the authors said.
Lead researcher Dr Dougal Hargreaves, of Imperial College London and the Nuffield Trust, said: "We know that there is already a growing crisis in the availability of Child and Adolescent Mental Health Services (CAMHS), with many more children and young people needing treatment than there are services to provide it.
"Our study suggests that this need is likely to continue to grow in future.
"Without more radical action to improve access to and funding for CAMHS, as well as a wider strategy to promote positive mental health and wellbeing, we may be letting down some of the most vulnerable in society."
He added: "The increase in reports of long-standing mental health conditions may also mean that children and young people are more willing to open up about their mental health, suggesting that we have made some progress in reducing the stigma associated with mental ill health."
Data from more than 140,000 participants aged four to 24 years old, from 36 national surveys, was analysed as part of the study.
Those aged 16 to 24 were almost 10 times as likely to report a long-term mental health condition in 2014 than in 1995, with the proportion rising from 0.6% to 5.9%, the study found.
The proportion of four to 24-year-olds in England who reported having a mental health problem increased from 3% in 2008 to 4.8% in 2014.
In Scotland, which also had comparable data for this period, the proportion grew from 3.7% to 6.5%.
In Wales, 2.9% of four to 24-year-olds received treatment for a chronic mental health condition in 2008, compared to 4.1% in 2014.
The authors wrote: "There has been a striking increase in the reported prevalence of long-standing mental health conditions among children and young people in England, Scotland and Wales in recent years, over a period of progressive improvement in many physical and general health measures."
Dr Max Davie, officer for health promotion for the Royal College of Paediatrics and Child Health, said: "As clinicians working alongside mental health services, we have known for some time that there is huge demand placed upon them.
"With the publication of this study, there is now further evidence of this, it must act as a catalyst for Government to take swift action."
He added: "As this study highlights, more children are talking about mental health, showing the stigma is starting to shift, but without the services to support growing patient numbers it brings, children are left with nowhere to turn."
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