Bullies are increasingly using the internet to terrorise teenagers outside of school, a survey suggests.
More than 10% of UK teenagers said they had been bullied online, while 24% knew a victim, the MSN/YouGov survey found.
The increasing popularity of instant messaging services and e-mail among children means bullies can now reach their targets at all hours.
Up to half of parents are unaware about online bullying, the survey of 518 children and parents said.
Changes in the way teenagers socialise both at school and away from the playground have been reflected in new forms of bullying, the MSN report showed.
Many teenagers use instant messaging (IM) services several times a day or regularly update a blog, redefining the conventional understanding of personal space and privacy.
Some 44% of those asked knew someone who had been threatened through e-mail or IM services, such as MSN and Yahoo.
About one-third knew of instances where bullies hacked into mail or IM accounts and sent embarrassing material from them.
And 62% of respondents knew about rumours or malicious gossip being spread online.
The potential for widespread humiliation among other online users drives the fear of online bullying, the MSN report said.
Nevertheless, teenagers are often slow to report bullying because of fears that parents could stop them using the internet.
The chairman of the Children's Charities Coalition for Internet Safety, John Carr, said that online bullying could seriously affect young people.
"The effects of this bullying can impact everything from morale through to school work and friendships as teens can end up isolated from or even afraid of the communications tools they use daily," he said.
Natalie Mead, head of citizenship for MSN UK, said the company was taking steps to ensure young people feel safe and protected online.
"We want everyone to be able to use our IM and blogging services positively and without fear.
"The same rules apply to protecting yourself in the online and real world environments: protect your identity and any information you regard as personal and don't let bullies get away with it by suffering in silence."