The Duke of Cambridge has urged everyone to be ready to challenge bullying behaviour in a video message to support the first ever National Stand Up To Bullying Day.
William warned that bullying is not just confined to the playground or classroom and can affect anyone regardless of age, background, gender, sexuality, race, disability or religion.
The Duke said it was "our collective responsibility to be alert, and to be ready to challenge the behaviour we see around us". He added: "We all have a role to play to ensure that we do not stand by, but instead stand up, and put a stop to bullying."
National Stand Up To Bullying Day has been spearheaded by The Diana Award charity, which was set up in memory of William's mother Diana, Princess of Wales.
The Duke recorded his message at Kensington Palace in May after hosting a round table discussion about the issue with Diana Award recipients and supporters.
He also highlighted the problem of cyber bulling.
"Bullying is an issue which can affect any one of us, regardless of age, background, gender, sexuality, race, disability or religion. It can happen for many reasons, it is often stupid and cruel, and can take many forms.
"And the reach of technology means it can feel unrelenting, leaving the victim feeling attacked, powerless and isolated. For young people in particular, bullying can have a profoundly damaging and long-lasting effect."
William stressed that more than 16,000 young people in the UK are absent from school each year due to bullying.
He added: "To shatter their trust in relationships and faith in the world around them at such a young age can have a lasting impact on their mental health and state of mind."
But he also warned that adults were affected as well.
"It is important that we recognise that bullying is not just confined to the playground or classroom, and it does not only affect children and young people. It exists all around us - in people's homes, in their workplaces, and in their wider communities," the Duke said.
"Research shows that victims of bullying can become more susceptible to depression, anxiety and suicidal thoughts, and experience difficulties in maintaining healthy relationships.
"This cannot be allowed to continue. And it is why I am supporting The Diana Award Stand Up to Bullying Campaign. It is our collective responsibility to be alert, and to be ready to challenge the behaviour we see around us.
"We all have a role to play to ensure that we do not standby, but instead stand up, and put a stop to bullying."
Research by YouGov commissioned by The Diana Award has shown that 81% of people questioned reported that bullying was commonplace in school, with 64% believing it is widespread throughout society, and 56% reporting that bullying is commonplace at work.
The survey of 2,000 adults in Britain aged between 18 and 55 also revealed that over two thirds (67%) knew someone who had been bullied, but just under a third of adults (32%) said they had never challenged bullying behaviour.
A host of celebrities - including Sir Ben Kingsley, Will Poulter, Christopher Eccleston, Ben Cohen, and James McVey from The Vamps - have also appeared in a video as they physically stand up to signify their support for the campaign.
Tessy Ojo, chief executive of the Diana Award, said: "We're urging everyone to get behind Stand Up To Bullying Day today. Make a pledge to do something different, to act differently and to offer kindness, whenever and wherever possible, because being a bystander, simply makes us a part of the problem."
The Duchess of Cambridge was bullied as a teenager when she attended independent girls' school Downe House in Cold Ash, Berkshire, before leaving after two terms for Marlborough College.