A worrying number of teenagers are swapping intimate or sexually explicit photos, called "sexts", on their mobiles, police say.
The Child Exploitation and Online Protection Centre said it receives daily reports of harassment after private photos have been circulated.
Some "sexts" have ended up on forums used by child sex offenders, it said.
One in four 11 to 18-year-olds have received a "sext" by phone or email, according to charity Beatbullying.
The charity, which surveyed 2,094 young people in England, said 23% of messages came from a current boyfriend or girlfriend, 45% from friends and 2% from adults.
If it is two 17-year-olds and they are in a consensual relationship, the police will probably not prosecute. But people that sent on these images [could be held] for ill intent"
Helen Penn, Ceop
Helen Penn, from Ceop, said police were becoming increasingly worried about "people losing control of these photos".
"We are getting more reports of teenagers being bullied, called names and strung up in front of their whole school," she said.
"We can completely understand why young people want to post these images to each other. But you have to have 100% trust in the person you are sending it to.
"Even then, what if their friends get hold of the phone or it gets lost on a bus somewhere?" she added.
Emma-Jane Cross, chief executive of Beatbullying, said it was important that parents and schools understood the rise of "sexting" so sexual bullying could be stopped.
The charity said girls were often bullied into taking, and sharing, explicit pictures of themselves.
I've never really been able to live it down. People I have never met still shout and swear at me... I would just never do it again
Alex, 16-year-old "sexter"
A third of young people have been involved in a "sexualised" online chat, it added.
One 16-year-old, Alex - not her real name - told Newsbeat life became "hell" after she sent her boyfriend a private "sext" video during a night out.
"He left his phone on the side and his mates got hold of it," she said.
The video was distributed around her school and neighbouring colleges and Alex said she was forced to take time off from her studies.
"When I came back it was like hell," she said. "I've never really been able to live it down. People I have never met still shout and swear at me. It's been the biggest lesson, I would just never do it again."
But another teenager told Newsbeat there was nothing wrong with "sexting".
"I send pics all the time. If you are comfortable with your body, then why not?", she said.
In theory, teenagers could be arrested for taking naked photos of themselves or their boyfriends or girlfriends.
Although it is legal to have sex at 16 under British law, it is illegal to take, hold or share "indecent" photos of anyone under 18.
Helen Penn said in practice it is unlikely the British police would get involved in a consensual case of "sexting" because it would not be in the public interest, although there have been a number of charges brought in both Australia and the US.
But if grooming or sexual abuse of a minor was discovered, there would probably be a prosecution.
"When the police look at this kind of offence, they are going to take it in context.
"So if it is two 17-year-olds and they are in a consensual relationship, they will probably not prosecute those people.
"But it will look at people that sent on these images and hold lots of them for ill intent."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.