The use of new technology such as text messaging in order to bully children is increasing, researchers have said.
Emails and texts can deliver bullying messages
A survey over four years of more than 11,000 children found nearly 15% had received nasty or aggressive messages.
And researchers from York have seen a steady increase in children suffering from this form of "cyber-bullying", a Cardiff conference will hear on Friday.
Girls were more likely than boys to report being bullied by e-mail and SMS, the survey reported.
Nathalie Noret, of York St John's University College, part of the research team presenting the findings to the British Psychological Society meeting, said: "It is a form of indirect bullying like gossip, that spreads quickly outside of the playground.
"Acknowledging the high prevalence of this kind of bullying is important because most interventions in schools are based on the assumption that bullying is physical or occurs face-to-face.
"Teachers and parents need to realise that a child's mobile phone or computer isn't just a communications tool - it is also a way for a bully to reach children in their own home."