A 'toxic climate' of online bullying is causing thousands of children aged 10 and younger to become so depressed that they require medical treatment.
Campaigners say up to 10 per cent of schoolchildren are affected by some form of mental illness, leading them to being treated in hospitals and other NHS centres.
A total of 4,391 children aged 10 or below have been treated for depression, anxiety or stress in the past five years at two of Britain's biggest NHS mental health trusts.
The figures from South London and Maudsley trust and South Essex Partnership University Trust were revealed following a Freedom of Information request from the Daily Mirror.
They cover just two out of 60 mental health trusts in England alone - meaning that the true number of children affected is undoubtedly much higher.
Children's mental health charity YoungMinds estimates that 96,000 children aged between five and 10 suffer from an anxiety disorder, while 8,700 are seriously depressed.
Overall, more than half a million children and young people up to the age of 16 are affected by a diagnosable mental health disorder, according to the charity.
The problems are significantly worse for children in care, three quarters of whom suffer from behavioural or emotional issues.
YoungMinds has previously forecast that by 2020, 100,000 children could be hospitalised every year due to self-harming.
Charity spokeswoman Lucie Russell told the Mirror: "An increase in under-11s needing mental health services is a sad and very worrying indictment of the society we live in and the pressures children face.
"Every day we hear about the unprecedented toxic climate young people face in a 24/7 online culture where they can never switch off, where they experience constant assessments at school, bullying, sexualisation, consumerism and pressure to have the perfect body at a young age."
A survey by YoungMinds revealed that 34 out of 51 local councils had cut spending on mental health services for children and teenagers since 2010.
Former Health Secretary Alan Johnson called for an investigation into the new findings, describing the number of children requiring treatment as 'shocking'.
He told the Mirror: "It is clear children are increasingly having to receive NHS care because of cuts to child and adolescent mental health budgets."
Health minister Norman Lamb said: "Children's mental health is a priority for this Government and we are investing £54million over this Parliament into improving access to therapeutic treatments for children.
"I have always been clear that mental health must be treated with equal importance as physical health and it is totally unacceptable to disadvantage mental health when allocating local funds."
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