Mental health services are turning away one in five children who are referred to them for treatment, including youngsters who have been abused and neglected, a charity has found.

The NSPCC warned of a "time bomb" of serious mental health conditions after it emerged that more than a fifth of children referred to NHS services for treatment for mental health issues were rejected.

Figures from 35 mental health trusts in England revealed that of 186,453 cases referred to them by family doctors and other professionals, 39,652 did not receive help.

In six trusts where children who had problems associated with abuse or neglect were referred to child and adolescent mental health services (CAMHS), 305 of the 1,843 cases were rejected - one in six.

The NSPCC said the lack of support for such children could lead to serious long-term mental health problems as young people are not getting the help they need early on.

Peter Wanless, the charity's chief executive, said: "If children don't receive the right kind of help and support following a disclosure, the damage can last a lifetime and include post-traumatic stress disorder, depression or suicidal thoughts in adulthood."

Abused and neglected children were often denied treatment because their cases did not meet the "high clinical threshold" required at a CAMHS, the NSPCC said.

But while not all such children will have a diagnosable mental health problem, many still need therapy and support to help them through their trauma.

There are also concerns around the strict criteria for children to get this support. The NSPCC said that in some areas the requirements that have to be met before children can received assessment or treatment posed "significant issues".

Mr Wanless said: "Not addressing their needs early on is just creating a time bomb of mental health problems. Sadly, the availability of specialist services that meet the needs of abused children, when they need it, do not appear to have kept pace with this growth in understanding of the crime.

"There is a vacuum that needs to be filled and it needs to be a national and local priority.

"Child and adolescent mental health services are just one part of the jigsaw and it's clear the current range of support available does not meet the needs of many abused and neglected children.

"Often children who are suffering with the consequences of what's been done to them won't necessarily meet a medical threshold but the emotional and psychological fallout of their abuse can snowball and get more severe in years to come."

Nearly 100 calls were made every week last year to the charity's ChildLine service from children who had suffered mentally because of abuse.

While there has been a rise in awareness of abuse in recent years, reports of abuse has also soared.

Sexual offences against children recorded by police in England and Wales increased by more than a third in 2013-14, the NSPCC said, while referrals to social services are also on the rise.