Head teachers have demanded an end to the "totally unacceptable" increase in attacks and threats from angry parents.

The National Association of Head Teachers, which has more than 30,000 members, said it handled 18 incidents of violence and intimidation against heads last month alone.

One head teacher received death threats, while four incidents related to physical attacks by pupils and parents who use violence as "a first resort", the union said.

The warnings follow similar complaints from classroom teachers' unions in recent months and calls for tougher rules on pupil and parent behaviour.

Speaking at the NAHT annual conference in Telford, Shropshire, general secretary David Hart said: "The rising level of abuse, threats and assaults by parents towards our members is totally and utterly unacceptable.

"Although we are still talking about a small minority of parents, this is what is happening on the 'front line' far too frequently.

"Some parents are unwilling to pursue their complaints by using the existing procedures properly.

"They use violence or threatened violence as a first resort.

"Governors, local authorities and the police must take the strongest possible action to support head teachers when they are faced with not only threats to themselves but also to their families."

NAHT figures for March showed the union dealt with 18 cases involving assaults, threatened assaults or abuse from parents or their families.

In eight incidents, parents were banned from the school site, while three cases related to threats of serious assault from pupils.

The union said verbal abuse and threats "often" continued to be directed at head teachers in their own homes - even extending to include their family members.

The figures come as both Labour and the Tories promise to get tough on violence and disruption in schools, with discipline a major education issue in the General Election.

Labour has backed a "zero tolerance" approach to all forms of disruption including low-level bad behaviour.

The Tories have pledged to back heads who decide to expel unruly pupils by abolishing the appeals panels that parents can use to get their children reinstated.