This is a slight improvement in primary schools, where the re-advertising rate fell from a high of 28 per cent to 26 per cent this past year, according to the study carried out for the National Association of Head Teachers.
But schools in both sectors are still complaining of low numbers of applicants, and are being warned the situation could worsen after mandatory qualifications for heads are introduced.
The number of headship vacancies last year fell from 2,105 to 1,951, partly because of falling numbers of heads seeking early retirement. This should have helped ease the recruitment crisis. Yet across all schools, 25 per cent of posts had to be re-advertised and 18 per cent remained unfilled after a second advert.
In London primary schools, the figure was 40 per cent, said the study, conducted by Education Data Surveys and based on analysis of schools advertising in The TES and a nationwide database of teacher appointments.
Three-quarters of primary schools which reported not filling a vacancy cited the poor quality of applicants as a reason. Only 30 per cent of all primary schools were able to shortlist more than five candidates, while only 10 per cent of secondary schools received more than 40 applications for a headship post. Special schools received only eight applications per post.
Professor John Howson, who compiled the report, said: "In primary schools, and especially in special schools, the introduction of the National Professional Qualification for Headship as a mandatory qualification pre-appointment could cause serious problems with such small numbers of applicants, particularly in the London area."
The Government is making the NPQH mandatory for new heads from 2002.
David Hart, general secretary of the National Association of Head Teachers, said: "It has to be a matter of serious concern when nearly 20 per cent of schools cannot find a headteacher to fill their vacancies.
"Applicants of the right quality are not coming forward in too many cases. This does not bode well as the Government moves rapidly towards the implementation of a Green Paper which will require heads to manage the biggest cultural changes the teaching profession has ever seen."
The study also found that 73 per cent of all primary heads appointed last year were women, though in secondary schools, two-thirds of appointments were men.
National figures from the Department for Education and Employment show that women headteachers now outnumber their male counterparts.