Headteachers are now among the most stressed professionals in the country with pressure of work accounting for four out of every 10 days that they are absent.
Research for the National Association of Head Teachers suggests that only doctors have more days off because of stress-related illness.
Heads took an average of 10 days off work over seven months this year, according to a survey of more than 1,800 schools.
The NAHT said that stress-related absences among heads had increased by more than a quarter last year, partly as a result of greater pressure from the Government.
The Schools Advisory Service, the insurance company which carried out the poll, said heads' stress had reached "danger levels".
A total of 38 per cent of lost days were the result of stress, nearly twice the UK average of 20 per cent. Heads have caught up with doctors at 39 per cent.
Mick Brookes, NAHT general secretary, said the introduction of compulsory planning and preparation time for teachers and a new management structure had added to heads' work.
"The ever-increasing workload on our members is putting their health in danger," he said.
Stress was a bigger cause of absence than accidents (blamed for 9 per cent of missed days); maternity leave (7 per cent); back-pain (6 per cent); operations (4 per cent); colds and flu (3 per cent); and hospital tests (2 per cent).
Other illnesses, ranging from cancer to headaches, accounted for their remaining absences.
Mr Brookes said: "The plans in the Government's white paper to shut schools they think are failing within a year will only make the stress even worse."
He said that if ministers did not stop this "bullying" there would be a backlash and heads would leave the profession in protest.
The Teacher Support Network said the proportion of calls to its helpline from headteachers about stress had increased from 4.7 per cent in 20032004 to 12.5 per cent in 20042005.
A spokeswoman said the heads were worried about issues included their workload and problems tackling pupils' poor behaviour.
The Department for Education and Skills insisted it was supporting heads by improving their pay and helping them manage their work.
A spokesman said: "No government has done more to support heads.
"We recognise the challenges and responsibility that the job brings."
Teacher Support Line is 08000 562 561 for England and 08000 855 088 for Wales.
GLAD TO ESCAPE FROM WHITEHALL OVERLOAD
Roger Davies cannot recall another time when he has felt under such pressure.
The 59-year-old said the latest government initiatives meant he was glad to be retiring from Oldbury Wells school in Bridgnorth, Shropshire, in July after nearly 25 years as its head.
"Being a headteacher is a wonderful job and I wouldn't have done anything else in the world, but I am relieved I am going to miss out on the next phase of life for schools," he said.
"The proliferation of initiatives, such as TLRs (teaching and learning responsibility points) has made life very difficult. And it's frustrating when the central business of running a school has to take a back seat."
Mr Davies began teaching business studies at the age of 24, without any formal training, after two years as a stockbroker.
When he took over Oldbury Wells in 1981 he was, at 34, one of the youngest secondary heads in England.
Mr Davies said he was pleased that schools now had greater control over their budgets and lighter-touch inspections.
But he said that government initiatives, including school improvement partners and the education white paper, would make it difficult for others to stay as headteachers for as long.
"Headships should be long-term jobs," he said. "Schools are like oil tankers - they take time to turn around."