Children of ten are regularly having sex, and sending explicit pictures of themselves to classmates when they reach their teens.
Boys routinely send pictures of their genitals to girls as young as 13, who agree to boyfriends' demands to send revealing snaps of themselves or even be pictured carrying out sex acts.
Rather than use a picture of themselves, some girls trawl for child pornography online to find images to send instead.
Child abuse investigator Det Insp John Geden said: 'We are seeing children engage in sexual activity much younger. We now see junior school pupils have sex with one another quite regularly. Girls of ten are having sex.'
The problem was highlighted in a survey carried out among year 9 pupils by Michelle Barry, who works with youngsters as part of a Southampton rape crisis project.
She said: 'I was gobsmacked when I asked a class of 13-year-olds if they had ever sent naked pictures of themselves and not a single hand did not go up.
'What is most worrying is the fact young people do not identify this as a problem. For them, it is part and parcel of school life.'
Mr Geden, from Hampshire Police, said: 'It is a serious problem we are faced with and it is a hidden problem because we do not truly know the full extent of it.'