In a speech to the National Union of Teachers' annual conference, John Illingworth, a primary school head for 13 years, said serious mental health problems caused by stress had "changed my life".
Mr Illingworth, head of Bentinck Primary School in a deprived part of Nottingham, was speaking as the union voted for industrial action to reduce teachers' workload.
Choking back tears, he told the conference: "This workload motion is too late for me. I'm removing the stress by leaving teaching. I think I've been a good head and a good teacher, but my health comes first."
He won a standing ovation from the conference after telling delegates he had been through counselling and seeking professional advice. "I've tried all that," he said. "I now know the best way to deal with unmanageable stress is to remove the cause of it. I'm doing that by leaving. Enough is enough."
Mr Illingworth, who has been a headteacher of three schools, said he had tried to run his school "in a way that was consistent with the principles of the union". "I have always felt bullied by government. I have been affected by serious mental health problems," he said.
Reporting to hospital for one of several check-ups, he was told by doctors that teachers and school staff now formed the majority of their patients, he said.
Mr Illingworth, 55, was president of the union in 2001 and had planned to stand for the general secretaryship of the union two and a half years ago. He withdrew because of stress, although he kept the reason secret at the time.
He is married with two grown-up children. His wife, Jen, was at the conference with him yesterday. He is hoping to seek early retirement and is currently on sick leave.
Mr Illingworth said that after his speech at least 30 delegates had approached him to say that either they or a close teaching friend had suffered similar problems. He said he had wanted to make his statement to show that "mental illness happens to ordinary people in ordinary circumstances".
Judy Moorhouse, the current president of the union, said to him: "I want to thank you so much for the courage you've shown. It is appalling that it has to take somebody standing in front of this conference in order to get some understanding by the world outside of what is going on in school after school."
Delegates to the conference in Torquay warned that many teachers were still working more than 50 hours a week, despite an agreement with the Government that they were entitled to 10 per cent of their school day off from the classroom, to concentrate on marking and teaching preparation. Delegates at the conference voted to urge the union's executive to consider industrial action to secure more funds from the Government to finance the agreement.