Video report by ITV News Reporter Rebecca Barry
Only one in eight child sexual abuse victims in England come to the attention of authorities, a major new study has found.
As many as 450,000 children could have been abused in the two years to March 2014, but only around 50,000 cases were recorded in that time, the Office of the Children's Commissioner said.
As a result, around 85% of sexually abused youngsters are not receiving vital intervention to keep them safe and overcome their experiences.
The report, released on Tuesday, said the prevalence of child sexual abuse in England has been vastly underestimated and that the majority of victims remain unidentified because services to protect them rely on children to speak out.
The prevalence of child sexual abuse in the family is such that it should now be recognised as a "national priority", the inquiry found.
It called for children as young as five to be given lessons at school to teach them about relationships and encourage them to discuss any concerns.
Simon Bailey, national police lead for child protection and abuse, warned that because children can easily access pornography, a generation "living out" what they see is being created.
He said: "I have had cases whereby 12,13-year-old boys are abusing four, five-year-old girls because what they have seen online they just thought was normal behaviour."
Child sexual abuse is thought to cost the UK economy £3 billion a year, the NSPCC said. Previous research has estimated that 1.3 million children in England will have been a victim of "contact" child sexual abuse by the time they are 18.
The inquiry, carried out using data from every police force in England, focused on abuse in the family environment, which it said accounts for two-thirds of all child sexual abuse.
Abuse was most likely to occur at the age of nine, but victims often did not speak out until they were adolescents.
Many did not recognise they had been abused until they were older and children did not speak out because of guilt and blaming themselves, fearing the person who abused them, the consequences of reporting it or due to loyalty to family members.The shame of the abuse being discovered and the fear of family breakdown was also a factor.
A quarter of the perpetrators of child sexual abuse were found to be children themselves, while 75% of the victims were girls.
Anne Longfield, the Children's Commissioner for England, called for urgent action to deal with the situation.
She said: "In recent years the terrible experiences of sexual abuse that some children have suffered in institutions or at the hands of groups of perpetrators have come to light, and preventing and tackling these has been made a priority.
"We must now wake up to and urgently address the most common form of child sexual abuse - that which takes place behind the front door within families or their trusted circle."
The report called for a new strategy led by the Government to prevent child sexual abuse, to strengthen the responsibilities of those working with children and to ensure professional bodies work together more effectively to identify problems.
School children should be taught about healthy and safe relationships in compulsory lessons and there should be a "whole-school" approach to child protection, with teachers trained to recognise the signs and symptoms of abuse and act accordingly.
A Government spokesman said tackling child abuse was a priority and that the report's recommendations would be carefully considered.
"We set up the first ever cross-government child protection task force to overhaul the way police, schools, social services and others work together in tackling this abhorrent crime," the spokesman said.
"We have also invested an extra £100m to support vulnerable children and we are providing £7m for specialist services for victims of sexual abuse, including those for child victims of sexual abuse and exploitation."