Teacher mental health was highlighted as an increasingly worrying issue, with the National Professional Qualification for Headship (NPQH) singled out for creating leaders bent on using bullying tactics on staff.

Heads, the NUT conference heard, had a “statutory duty of care” towards staff, and mental health was an important health and safety issue.

Paul Bell, a headteacher from Medway who resigned due to stress, said: “I did the NPQH and it was appalling, but I didn’t come out of it as a Stepford head. The training is a charter for bullying. I burnt out and now haven’t worked for a year.”

The union has backed its claim that workplace bullying is increasing with a study which found more than 93 per cent of divisional secretaries thought casework relating to inappropriate management in schools had increased in the past five years.

The report, entitled Reign of Terror, also found that cases of inappropriate management were more likely to come from schools where the head had been in post for fewer than five years.

Schools “failed” by Ofsted were also more likely to see staff turning to their unions with complaints about the leadership.

But one teacher complained that the opposite situation could also create stress.

“There is a lot of pressure on staff if the school is outstanding,” they said.

The report also expressed concerns over the additional pressures created by KS2 Sats, the growth in lesson observations and “school improvement staff” sent by local authorities to boost results.