More than one in seven girls and one in 20 boys in England and Wales are sexually abused before they turn 16, according to a Home Office-backed investigation.
It said the majority of abuse is never reported or uncovered, and it is often not until the child is an adult that they may speak out.
Experts from the centre of expertise on child sexual abuse (CSA Centre) said the abuse had become more hidden and that the gap between recorded cases and levels of abuse was likely to be widening.
The CSA Centre, which is funded by the Home Office, analysed prevalence estimates and official data up to March 2020 for the report.
It found that estimates vary, with 7.5 per cent of adult respondents to the 2019 Crime Survey for England and Wales (CSEW) reporting experience of child sexual abuse before turning 16 – a similar proportion to those who experienced physical abuse.
A 2009 NSPCC study found that 24.1 per cent of UK 18 to 24-year-olds reported experiencing child sexual abuse up to age 18. Based on these and other data, the CSA Centre suggested a more realistic estimate is that at least 15 per cent of girls and five per cent of boys experience some form of CSA before the age of 16.
This includes contact abuse in all forms, including acts by those under 18, as well as indecent exposure. Existing data does not show how prevalence has changed over time, how much child sexual abuse is committed by under-18s or how much is facilitated online.
Fewer than 10 per cent of children become known to agencies when abuse is taking place, according to the CSEW.
Since the period examined in the report, the coronavirus pandemic response is likely to have increased some of the known preconditions for child sexual abuse and limited professionals’ abilities to respond to concerns, said the report.
It said: "In the year to March 2020 the overall picture was one of a continuing and significant gap between the number of children who experience sexual abuse and the much smaller number who are identified and supported by agencies.
"It is likely that the Covid-19 pandemic widened this gap further in 2020/21, with lockdowns making children even less likely to be able to be noticed or tell someone about their abuse."
Ian Dean, director of the CSA Centre, called for better data, a national prevalence study and dedicated training to help professionals such as teachers, social workers and the police respond to concerns.