One in five adults in England and Wales experienced abuse before they were 16 years old, according to a report by the Office for National Statistics (ONS).
The ONS estimates that 8.5 million people aged between 18 and 74 were abused or witnessed abuse as children.
Around 14% of adults who sought help from the National Association for People Abused in Childhood (Napac) last year had not told anyone before.
The report studies the emotional, physical and sexual abuse of children.
It was compiled using data from the Department for Education, the Welsh Government, Napac, and the National Society for the Prevention of Cruelty to Children (NSPCC).
It also shows that at the end of March last year, more than 54,000 children were in the care of local authorities in England and Wales because they had experienced or were at risk of abuse.
In the year leading up to March 2019, Childline delivered 19,847 counselling sessions to children in the UK where abuse was the main concern.
That is a slight fall from the previous year, when 22,133 counselling sessions were given for this reason.
The findings on adults experiencing abuse as children were compiled using data from the Crime Survey for England and Wales (CSEW).
It includes people who witnessed domestic violence.
However, the CSEW underestimates the total number of adults who were abused as children because it does not count abuse against 16 and 17-year-olds.
The ONS report also found that half of adults (52%) who experienced abuse as children also experienced domestic abuse later on, compared with 13% of those who did not.
Around 46% of women who were abused before the age of 16 experienced more than one form of abuse, compared with 41% of men.