The shocking statistic came as ministers launched a campaign against victimisation via websites, text messaging and emails. They say it is the most insidious form of bullying as it can follow victims everywhere, even into their bedroom.
The Government has commissioned new guidance for schools on how to tackle the problem and an internet campaign aimed at preventing the forwarding of abusive messages and videos.
Tomorrow the Department for Children, Schools and Families will also launch new guidance on homophobic bullying. It hopes to create a "sea change" and make it as unacceptable in schools as racism.
The department's survey of 1,000 12- to 15-year-old pupils found that 34 per cent had been cyber-bullied. It will use free online advertising with pop-ups and video clips provided by the Yahoo, MSN, Bebo and MySpace websites, targeted at the "middlemen" who are helping to fuel the problem.
"It is aimed at the people who forward and download this material but don't realise they are part of the problem and making the bullying worse," said a DCSF spokesman.
"We are trying to say that if you get sent this material and you forward it on, then you are a bully as well. You are creating an audience for the bully and making it much worse for the victim.
"If you receive these emails and messages then just press delete."
The cyber-bullying guidance is being backed by all teacher unions as well as major websites and mobile phone companies.
It says schools should look at confiscating mobile phones, make sure computers are in public areas and that their use is monitored. It also recommends that pupils should avoid putting their personal details online. A DVD showing actors being bullied is also being sent out to teachers.
The campaign came in the week during which the Liberal Democrats' annual conference in Brighton voted for tougher anti-bullying measures. They include requiring schools to address homophobic, gender, racist, disability and "faith-based" bullying.
The party wants all schools to have a trained staff member to counsel victims, a governor or member of the senior management team with specific responsibility for anti-bullying policy and an accurate database of bullying incidents.
Stephen Williams, the party's shadow schools spokesman, said specific policies were needed for homophobic bullying because victims may not have come out to their parents or have a peer group to support them.