A quarter of girls and a sixth of boys under 16 in Britain could be victims of sexual abuse, an expert has said.
There could be as many as 11 million victims of sexual abuse in the UK - constituting a "national health epidemic," Graham Wilmer, founder of the Lantern Project that supports victims of sexual abuse, said.
He added his estimation was based on "prevalence rates published by the government".
Mr Wilmer, an abuse victim himself, told Sky News: "There are potentially about 11.7 million victims out there at the moment who have not disclosed, and many of those people will start to come forward in very significant numbers."
"You are dealing with a massive, massive problem. From what we have seen, if you don't provide the right level of support and intervention to support people when they come forward you see very significant health problems, mental health and physical health, which have a direct cost to us as a society.
"We look upon child abuse and it's impact now as a national health epidemic."
Mr Wilmer was appointed by Theresa May as an independent panel member on the controversial historical sex abuse inquiry, which has already seen the resignation of two chairs and anger from some victims themselves.
According to Sky News, one in six boys under 16 have been sexually abused - for girls the figure is one in four.
Meanwhile, children's charity the NSPCC estimates one in 20 children in the UK have been sexually abused.
Mr Wilmer added: "All of the forces of law, and the forces of support work that are available are all gearing towards saying this number is a big number, whether it is 11.5 million or 11.1 actually isn't really relevant.
"The fact is that it is a massive number and because it is a massive problem we are now focusing on it and we are going to do something about it."
The Government's inquiry has been dogged with problems, mainly around finding a chairman, since it was announced in July.
Mrs May has apologised to victims for failing so far to find a suitable person to fill the role.
Baroness Butler-Sloss stood down as chairwoman in July amid questions over the role played by her late brother, Lord Havers, who was attorney general in the 1980s.
Her replacement Fiona Woolf, the then Lord Mayor of London, resigned in October following a barrage of criticism over her ''establishment links'', most notably in relation to former home secretary Lord Brittan.
And last week a number of alleged victims of child sexual abuse have said they will withdraw from the inquiry into the issue unless the Government makes major changes to it, including extending the period of time it will cover to further back than 1970.