Boxing champion Joe Calzaghe said he was deeply affected by bullying he suffered between the ages of 13 and 14.
Retired world champion boxer Joe Calzaghe has described how school bullies turned him into a "introverted wreck" as a youngster.
He was speaking before launching an advertising campaign for Beatbullying, a charity helping victims of the form of abuse across the UK.
Calzaghe, the former world super-middleweight champion, quit boxing last month after 46 unbeaten fights.
He said bullying online now created more misery for many children.
The 37-year-old from Newbridge, in south Wales, was helping Beatbullying launch an advertising campaign for its new website CyberMentors.
The social-networking site is staffed by more than 700 youngsters who offer advice to their peers about bullying.
I went into a shell, my personality changed. I used to hate going to school
Since it was launched on 3 March it has been visited by more than 23,000 young people, said Beatbullying.
It said hundreds of those who had sought help admitted having suicidal thoughts or wanted to self-harm as a result of bullying.
Calzaghe is patron of Beatbullying.
He said: "I know only too well from my own experiences as a victim of bullying while at school what a serious issue bullying is for thousands of youngsters.
"For two years I was bullied, called names and ignored by former friends which turned me from a happy, out-going kid who enjoyed school and schoolwork, into an introverted wreck, detached from his studies and scared of his own shadow during school hours.
"Unfortunately, in the age of the internet - which didn't exist in my school days - cyber-bullying has now added to the misery bullies can inflict.
"If I can take my own experience and raise awareness and help because of what I've achieved in boxing then I will be happy."
He added that he believed the website would empower youngsters to stamp out bullying.
"I've been through it myself. It's a very horrible thing to go through especially as a child at school," said Calzaghe.
"A lot of the other kids who I didn't really know would gang up and call me names and that happened for over a year.
"It was a very tough time for me. I bottled a lot of it up inside. It just goes to show if it can happen to me it can happen to anybody, it's nothing to be ashamed of or embarrassed about.
"I went into a shell, my personality changed. I used to hate going to school."
He added: "I was the world champion for 11 years, I was the best fighter in the world, but I was also somebody who was bullied - it just goes to show it can happen to anyone.
"A lot of kids do self-harm and do kill themselves and maybe if they had someone to talk to this wouldn't happen."
Beatbullying adverts will be posted on more than 1,000 billboards and bus stops across the country and will also feature in magazines and on websites.
They have been designed for free by M&C Saatchi and depict the silhouettes of young people who have killed themselves, next to words commonly used as taunts.
Beatbullying said that in a survey of 2,000 youngsters aged between 11 and 18, 56% admitted to being bullied online.
Emma-Jane Cross, chief executive of Beatbullying, said: "Bullying in any form is unacceptable but sadly it is an issue that has only been propagated by digital innovations."
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