The prevalence of child sexual abuse in England has been vastly underestimated after a major new study suggested only one in eight victims come to the attention of authorities.
Around 50,000 cases of sexual abuse were recorded by police and local authorities in the two years to March 2014, the Office of the Children's Commissioner said.
But the true number of children abused in that time is thought to be as many as 450,000, meaning around 85% of sexually abused youngsters are not receiving vital intervention to keep them safe and overcome their experiences.
The majority of victims remain unidentified because services to protect them, including police and social services, rely on children to speak out, the report 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, the national police lead for child protection and abuse, warned that the ease with which children can access pornography through technology was creating a generation who are "living out" what they see.
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 for years, until they were adolescents.
Many did not recognise that 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 or the consequences of reporting it or loyalty to family members.
Other barriers included the shame of the abuse being discovered and the fear of family breakdown.
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 prevent abuse, identify it early and provide better support for victims.
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."
Better detection is key to helping prevent abuse, the inquiry found.
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.
Victims of child sexual abuse should also receive support specific to their needs, and government should review how agencies work together when investigating abuse to ensure children are not traumatised even further.
The prevalence of child sexual abuse in the family is such that it should now be recognised as a "national priority", the inquiry found.
The cost of policing allegations of child sexual abuse in England is around £1 billion a year, said Mr Bailey, but Ms Longfield said "the cost to public services of not acting is immense".
New investment is needed, but will save on social economic and social costs in the long-term, she said.
Child sexual abuse casts a "long shadow" with an impact that "can last a lifetime", Ms Longfield said, adding that the goal was for a "major reduction" in the number of children being harmed in the next five years.
Jon Brown, the NSPCC's lead on tackling child sexual abuse, added: "The legacy of child sexual abuse, whether it is within the family environment or whether it is outside, is absolutely huge.
"The question needs to be asked, can we afford not to address it? We have estimated that the annual cost to the UK economy of child sexual abuse is over £3 billion a year."
A Government spokesman said: "We are determined to get the very best to the front line of social work and tackle failure.
"That is why this government has made tackling child abuse a priority - 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.
"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. We will carefully consider the recommendations in the report."
Roy Perry, chairman of the Local Government Association's children and young people board, added that councils are offering child protection to around 20,000 more children than seven years ago, suggesting more abuse is being identified.
He said: "While this is encouraging, we also need to be sure that victims are able to access the support they need once abuse is identified."
Gabrielle Shaw, chief executive of Napac (National Association for People Abused in Childhood), added: "We need to invest in preventing abuse in the first place, in more effective partnership working between agencies and having a more child-focused approach in dealing with cases of sexual abuse in the family environment."