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 research studies emotional, physical and sexual abuse from threats and belittlement to beatings and rape.
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 called one charity helpline last year had not told anyone before.
The research was compiled using data from the Department for Education, the Welsh Government, the National Society for the Prevention of Cruelty to Children (NSPCC) and the National Association for People Abused in Childhood (Napac), which runs the helpline.
It aims to provide a better understanding of the scale and nature of the abuse of children.
There is no single definition of child abuse, and the report includes a variety of offences or negative experiences.
It includes sexual abuse, such as rape and other assaults like indecent exposure; physical abuse, such as throwing objects and smacking; and emotional abuse, including behaviour that suggests a child is not loved.
The report also takes into consideration people who witnessed as children any of those forms of abuse.
It 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.
By Tom Symonds, Home Affairs correspondent
What's most significant about today's research is that it suggests 8.5 million people have not only experienced abuse, but could be living with the life-long effects of having experienced it.
Sexual abuse, in particular, can have an impact throughout adulthood - resulting in failed relationships, involvement in crime, depression and suicide.
Any one of the victims may need help later in life, and experts working in the field increasingly talk about the need for what they call "trauma-informed" support.
In other words: help for adults that takes into account the experiences they had as children.
As for preventing the abuse of children, this research could add to the pressure for so-called mandatory reporting, making it a legal duty for people working with children to report evidence of abuse.
The ongoing public inquiry into child sexual abuse looks increasingly likely to support it.
The findings on adults experiencing abuse as children were compiled using data from the Crime Survey for England and Wales (CSEW).
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. Of those who did not experience abuse as children, the figure was 13%.
The research also reveals:
- 3.1 million people - 7.5% of adults - experienced sexual abuse as children.
- Around one in four women experienced abuse before the age of 16 compared with one in six men.
- 32% of CSEW respondents who had mixed or multiple ethnic identity experienced abuse as children, compared with 21% of white respondents, 17% of black respondents and 11% of Asian respondents.
- Children who lived with adults with a long-term health condition, disability, mental health condition, problem drinker or drug user, were more likely to have experienced abuse.
- 52,260 children in England were the subject of a Child Protection Order in March last year.
Alexa Bradley, from the ONS Centre for Crime and Justice, said: "Child abuse is an appalling crime against some of the most vulnerable in society, but it is also something that is little discussed or understood.
"Today's release is ONS's first attempt to fill an important evidence gap on this critical issue."
NSPCC spokesman Andrew Fellowes said the report shows how the "devastating effects" of child abuse can impact victims in adulthood, but said that it is still not clear how many children "are suffering right now".
"It's crucial government conducts a prevalence study so we get a true picture of the scale of abuse in the UK," he said.
"Only then will we know what services are needed to protect and support abused young people."
If you or someone you know has been affected by these issues, help and support is available at bbc.co.uk/actionline