Almost 2,000 primary heads are expected to quit this year, with London schools finding it increasingly difficult to recruit replacements.
Nearly a third of the inner London primary schools with vacancies this year have had to re-advertise posts. Elsewhere, 14 per cent of the vacant primary headships and 9 per cent of secondary posts have had to be re-advertised.
The turnover of headteachers in secondary schools this academic year is set to equal a six-year high.
The job losses come amid rising class sizes, with Government figures this week revealing that pupil:teacher ratios in primary and secondary schools this year are worse than a decade ago.
From September to May 17, 395 secondary and 1,803 primary headships have been advertised in The TES. There have been 540 deputy vacancies in secondaries advertised, and 1,513 in primaries.
The figures do not include independent schools, and some advertisements may have run for two weeks. But on the face of it, they would indicate an 8 per cent increase in secondary headteacher vacancies in 12 months. This comes on top of a 13 per cent rise in the previous year.
John Howson, from Oxford Brookes University, who conducts an annual survey of headteacher vacancies based on TES advertisements, said : "All the indicators are pointing in the wrong direction for headship appointments."
He warned that parts of inner London were facing crisis, and said problems were now also hitting special and independent schools.
Mr Howson, senior lecturer in the school of education, said 9 per cent of the secondary headships advertised in the first five months of this year had been re-advertised.
His research has also revealed that 483 primary headships were advertised in January and February - 38 more than in the same period in 1995. The re-advertisement rate was 14 per cent, compared with 15 per cent in 1995, but Mr Howson said: "I expect that figure to rise."
He added: "We are heading back to the situation of the late 1980s, when 40 per cent of primary headships in London failed to get filled."
Six out of every 10 London heads who are leaving are going early, according to the National Association of Head Teachers. It estimates that for every head retiring early, there are two or three just waiting to reach 50 in order to follow suit.
The NAHT has so far identified 183 London heads leaving this academic year Q 42 retiring early for medical reasons (usually stress-related), 19 "under duress", and 12 being made redundant.
Delegates to the NAHT's annual conference in Torquay next week will hear how difficult it is to attract applicants to posts, and there will be a call for spot salaries.
Reasons for heads quitting have ranged from impossible workloads, financial cuts and changes brought about by Government legislation to public criticism following poor inspection reports.
David Hart, NAHT general secretary, said: "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 schools of the capital city."