Over 100,000 children have been rejected for treatment by local mental health services after being referred in the last two years.
A Freedom of Information request by the NSPCC to NHS Trusts in England revealed that on average 150 children each day were turned away from NHS children's mental health services between 2015 and 2017.
The charity is calling upon the Government to focus on early intervention to help children in order to reduce the numbers who reach crisis point.
From a total of 652,023 cases referred to child and adolescent mental health services (CAMHS), 109,613 children were turned away.
The NSPCC fears the number of young people being dismissed from mental health services could be significantly higher, as one in five of the Trusts which responded to the request failed to disclose the number of rejected referrals.
Research shows that half of mental health conditions experienced by adults first develop before the age of 15, the charity says early intervention is key to minimising long-term impacts.
Last month it emerged that the NSPCC's Childline delivered 63,622 counselling sessions to children reporting suicidal feelings in 2016/17.
The findings also painted a worrying picture of a postcode lottery of waiting times, with the average waiting time ranging between just two days in some areas to more than five and a half months in others; and an average waiting time of eight weeks.
Peter Wanless, NSPCC chief executive, said the situation is putting children at risk: "It is desperately sad to see so many young people facing distress around mental health issues being forced to wait months for assessment by CAMHS, many of whom are then rejected for treatment altogether.
"This risks leaving them in limbo while their condition potentially reaches crisis point," he added.
Mr Wanless also said that while the charity recognises the hard work of mental health professionals in trying to help young people, too many children who need help are struggling to get help.
"The Government's upcoming Green Paper on mental health must urgently evaluate the early support systems available to young people to ensure that no child is left to suffer in silence."
Previous NSPCC research has found that an increasing demand for services means many young people, including those who have suffered sexual abuse and neglect, do not meet the clinical threshold to access CAMHS and are turned away from treatment.