Anonymity online can be "really, really dangerous" as it allows cyberbullies to ignore the real-world impact of their actions, the Duke of Cambridge has said.
William highlighted the "human tragedy" of online harassment such as suicide in a video launched at the start of a new campaign against cyberbullying.
The Royal Foundation's Taskforce on the Prevention of Cyberbullying is bringing together organisations including Facebook, Snapchat and Google to combat the issue.
William's interest in the problem began shortly after the birth of his own son, Prince George, when he heard the story of a boy who killed himself as a result of online abuse.
A video released by Kensington Palace shows William meeting Lucy Alexander, whose son Felix also killed himself, and cyberbullying victim Chloe Hine.
Ms Hine attempted to take her own life at the age of 13 after enduring an onslaught of abuse on the internet.
Listening to the experiences of the pair, the Duke tells them: "I think it is worth reminding everyone what the human tragedy of what we are talking about here isn't just about companies and about online stuff - it's actually real lives that get affected.
"And the consequences, that is the big thing, the consequences of what happens if things are not kept in check in terms of what we say and what we do.
"We are still responsible for our own actions online - this anonymity, as you were saying, is really, really dangerous."
Ms Alexander spoke of her son's slide into depression after he was targeted on social media, being excluded from parties and viewing himself as "stupid and ugly".
She said: "It just ate away at him inside, I think, but I had no idea of the depth of his despair at all."
The Duke told her: "It is one thing when it happens in the playground and its visible there and parents and teachers and other children can see it. Online, you're the only one who sees it, and it's so personal, isn't it? Really it goes straight to your bedroom."
Ms Hine escaped her own personal torment by turning to writing to help her process her feelings.
William asked to hug both women after they shared his stories at Kensington Palace, thanking them for their "brave" intervention.
The Taskforce will bring together charities, tech organisations and media organisations to work alongside young people in an effort to tackle cyberbullying.
It is chaired by entrepreneur and founder of travel website lastminute.com, Brent Hoberman.
Other members include The Anti-Bullying Alliance, Apple, BT, The Diana Award, EE, Internet Matters, NSPCC, O2, Sky, Supercell, TalkTalk, Twitter, Vodafone and Virgin Media.