At the PAT annual conference in Harrogate, delegates heard that bullies have posted mobile phone videos on websites, showing teachers as well as pupils being attacked or humiliated.

They backed a motion demanding that such websites be closed down.

It was proposed by Kirsti Paterson, from the PAT's Highland and Western Isles Federation, who said one teacher had been the subject of a death threat which was posted online.

She said a pupil posted a doctored picture of the teacher, headless, with the caption 'You are dead'.

She added: "In the short term, confronting this problem must be the closure of sites encouraging cyberbullying."

Catriona Tait, a newly qualified primary teacher from Dundee, said one in four pupils had experienced online and text bullying at some time.

She said: "Not only are the children and young adults in our care subjected to such torment, it would now appear that members of the profession are becoming victims of cyber-bullying.

'Important issues'

"Nobody should be subjected to such humiliation anywhere, let alone in their place of work."

A spokesman for the website YouTube said: "YouTube is a community site used by millions of people in very positive ways.

"It's also used by organisations such as Greenpeace and Friends of the Earth to reach people on a range of important issues.

"Sadly as with any form of communication, there is a tiny minority of people who try to break the rules."

He said YouTube had joined the UK government's bullying task force, working closely with teachers unions and others.

Emma-Jane Cross, chief executive of the charity Beatbullying, rejected the idea of closing down websites.

She said: "Calls for social networking sites like YouTube to be closed because of cyberbullying are as intelligent as calls for schools to be closed because of bullying.

"Cyberbullying is flourishing for two reasons. First, society is not adequately preventing bullying behaviour. Instead we celebrate it on television programmes.

"And secondly, it seems to be easier to type something hateful to a school friend rather than say it to their face."

A spokesman for the Scottish Executive said: "Bullying of any kind is unacceptable. The executive is committed to supporting schools, authorities and communities to prevent and tackle bullying whenever and wherever it occurs.

"As part of this support, we fund Scotland's anti-bullying service Respectme, which offers specific advice to parents and teachers on cyberbullying."