Thousands of young people are committing acts of sexual abuse against other children every year, the NSPCC warned.
The charity found there were more than 5,000 cases of abuse by under 18s reported to the police in the last three years. In some instances acts of sexual abuse were committed by children as young as five or six.
Nearly all - 98% - of the 4,562 offenders were boys and where the relationship was recorded, at least three out of five of the victims knew their abuser, the NSPCC said. More than a third of the offences were said to have been committed by a family friend or acquaintance, and one in five by family members.
The NSPCC obtained the statistics through Freedom of Information requests to each of the 43 police forces in England and Wales. But only 34 forces supplied figures - revealing a total of 5,028 offences - so the true number of offences is likely to be higher, the NSPCC said.
The findings follow a report by probation inspectors last month which found that police, social workers and teachers were missing the warning signs that a child may sexually offend.
The NSPCC warned that easy access to indecent material could be leading to an increase in the number of children needing help. The charity has found that more children were carrying out online grooming and harassment.
Claire Lilley, policy adviser at the NSPCC, said she hoped the findings would ring "alarm bells" with authorities that the problem required urgent action. She said: "In some cases older children are attacking younger ones and in other cases it's sexual violence within a teenage relationship. While more research needs to be done on this problem, we know that technology and easy access to sexual material is warping young people's views of what is 'normal' or acceptable behaviour."
A Government spokesman said: "The number of young people cautioned or convicted for sexual offences has fallen by nearly a quarter over the past five years. However, these young people remain some of the most challenging in society and most have extremely complex issues and needs.
"We are driving up the skills and experience of social workers so they are better able to identify the warning signs much more quickly as well as strengthening guidance on child protection."
Anyone worried about a child can contact the NSPCC on 0808 800 5000, and children wanting help can call ChildLine on 0800 11 111.