The dramatic rise has cost the union Pounds 1 million in legal and other fees associated with defending members. The NAHT has 68 cases of suspension and David Hart, its general secretary, blames “trigger-happy” governors who suspend first and ask questions later. Another factor, he said, is the increasing stress.

Of the 68, 17 have been suspended on grounds of competence, 31 for alleged misconduct, and 20 for ill-health. In the latter cases, governors have told the head to stay away because of medical unfitness, often caused by disputes over competence. Under 10 are deputies.

Suspension - which can last from a fortnight to two years - often follows complaints from parents or staff or a breakdown in relations with the governing body. Misconduct claims include alleged sexual abuse, improper behaviour with girl pupils, sexual harassment, affairs with pupils’ parents, and financial irregularities.

Mr Hart said heads and teachers have been particularly vulnerable to allegations of indecent assault since the Children Act was introduced. Police and social services are taking a more active line on allegations, sometimes mischievous, of abuse. He said he hoped guidelines agreed by the unions, the Department for Education and the Home Office will offer more protection.

“We will also be taking up Mrs Shephard’s invitation to see her about the problems some heads and governing bodies are having, and the need for a code of practice for the governance of schools,” he said.

The union believes that school inspections and increased responsibilities are causing stress, sometimes resulting in breakdown and even suicide. The NAHT has set up a stress helpline for its members.

At its annual conference in Harrogate, Mr Hart said that the average primary head’s salary of Pounds 28,000 and average secondary head’s Pounds 40,000 was not enough to reward his members.