YouTube and other video-sharing websites have a "moral obligation" to tackle cyber-bullies, according to UK education secretary Alan Johnson.

Johnson will today call for sites such as YouTube and RateMyTeachers to remove offensive videos of people being attacked or harassed.

The education secretary is expected to tell the National Association of Schoolmasters and Union of Women Teachers conference in Belfast that bullying is causing many teachers to leave the profession.

The move comes five months after the NASUWT demanded action to stop pupils humiliating them by posting offensive video clips and abuse online.

NASUWT general secretary Christine Keates said: "Bullies are being handed an increasingly sophisticated tool with which to make life a misery."

Johnson will also use today's conference to outline new powers for teachers to confiscate mobile phones used for abusive calling and messaging.

He will tell delegates: "Cyber bullying is cruel and relentless, able to follow a child beyond the school gates and into their homes. The online harassment of teachers is causing some to consider leaving the profession."

Johnson wants sites like YouTube and Rate My Teachers to "take firmer action to block or remove offensive school videos in the same way that they have cut pornographic content."

RateMyTeachers says that it moderates its content and insists that 70 per cent of its postings are positive. YouTube says it trusts its users to "be responsible, and millions of users respect that trust."