Heads say stress is particularly bad in London because of the need to take action against the above-average numbers of poor teachers and because of increasing threats from violent parents.
They also blame impossible workloads and lack of support from governors and local authorities.
Of the 183 London heads retiring this year, 110 are leaving early, 43 for medical reasons, usually related to stress. Nineteen have been forced out and 12 are leaving because they are redundant. Fourteen of those leaving because of ill health are under 50 and only four are over 60.
During the last four months, 54 London heads have asked the association for advice about early retirement. Eight of these have had breakdowns. Many have been in post for only a short period of time.
The figures represent a big increase on last year.
Brian Fuller, the association's regional officer, said: "Teacher competency is a particular problem. In the Eighties, it was very difficult to recruit anybody in London. A lot of people were appointed who should not have been.
"It is down to heads to take disciplinary action. Some authorities don't back heads up in the action . A lot of governors also back down from dismissing teachers after the head has gone through agony."
More parents, he said, were threatening both teachers and staff and even vandalising their cars. "Since Christmas, I have been getting four or five calls a week from heads who want to get out of the profession."
The association said spending cuts, changes in legislation, increased public accountability and public criticism after inspection reports had all increased heads' load. The figures were simply the tip of the iceberg, it added: "For every head retiring early, there are two or three just waiting to reach 50 years of age in order to go."
Applications for London headships are declining fast. A study by Oxford Brookes University earlier this year showed that nearly 40 per cent of inner London headships were re- advertised. Deputy heads are reluctant to apply for headships.
One inner London secondary school appointed a head after four attempts but the head left after 12 months. A grant-maintained secondary school attracted only three applications. Even a large primary school in an affluent part of London had just six applications.
David Hart, the association's general secretary, said: "London schools can ill-afford to lose such a quantity of experienced talent. City schools often provide for the most vulnerable and disadvantaged pupils in the country. They desperately need stability and continuity.
"New ways must be found to encourage able, experienced heads and deputies to work and stay in city schools, otherwise this haemorrhaging of talent will set back efforts to raise standards in the schools of the capital city."
Mr Fuller said that the cost of housing in London made it difficult for teachers from outside the city to move into it.
The Government's chief schools inspector yesterday blamed headteachers and governors for not being determined enough to sack bad teachers. Chris Woodhead told MPs no extra powers were needed to get rid of the 15,000 teachers he had earlier claimed were harming children's education.Reuse content