One in five children has been threatened or bullied online, according to a survey released today.
The study also found more than a third have given out personal details including their email address to people they had met only online.
Professor Julia Davidson of Kingston University, who led the study, said the information should be a wake-up call to children and parents.
She said: "A significant proportion of teenagers engage in behaviour that adults would consider risky, such as posting personal information and photos of themselves.
"This is often viewed as acceptable by young people. We need to better understand teenagers' online behaviour in order to develop more effective policy and safety practices."
More than 1,700 youngsters aged between 11 and 16 took part in the research, which was commissioned by the Child Exploitation and Online Protection Centre (CEOP).
One in five said they had shared their full name, their school or a photo with someone they had met only online.
But nearly all the children questioned (96%) said they would not arrange to meet a person they had met online.
Prof Davidson said more training was needed to help parents teach youngsters how to use the internet safely and safety tips should be displayed in schools and reinforced by teachers.