The children, who contacted the charity with their stories, remain anonymous to protect their identities.

A 15-year-old boy spent a lot of time on the internet, and frequently visited chatrooms.

In one, he became friends with a person who supported the same football team, and they exchanged e-mail addresses and mobile numbers.

However, after a while they fell out, and he began receiving abusive e-mails.

He changed his e-mail address but forgot about his mobile phone number,

After a while he began receiving messages threatening to find out where he lived.

The sender of the messages threatened to beat him up, and finally threatened to kill him.

The boy told his parents, who alerted his mobile provider.

The company investigated but was unable to trace the sender, because the messages were being sent free from a website.

However, the firm agreed it was a case of harassment and gave the boy a new number.

Sexually explicit

In another case, a 12-year-old girl became very distressed after suddenly receiving sexually explicit messages.

Neither she nor her parents recognised the number from which the messages were sent, and when they rang it on several occasions, they received no reply.

The girl could not recall giving out her number to someone who would have sent such messages, or to anyone who would have passed her number on.

Her parents did not report the messages to the police, because they were worried that she would be upset and distressed by any subsequent interview.

Instead, they stopped using the phone and bought a new one with a different number.