Roman Catholic schools and those in London are bearing the brunt of a worsening crisis in headteacher recruitment, research shows.
Four out of 10 failed to appoint a head in the past year, according to a study commissioned by the Secondary Heads Association and National Association of Head Teachers, nearly twice the national average.
Nationally, more than a fifth of schools in England and Wales which advertised for a head in the past year failed to make an appointment.
Difficulties in appointing senior staff worsened for all schools, with 28 per cent of primary and 20 per cent of secondary schools reporting unfilled headships after the first round of applications.
Headteachers at Catholic schools, quoted anonymously in the report, suggested they suffered because they tended to demand practising Catholics.
St Mary's RC primary in Chiswick, London, has been advertising for a new head since January without luck, despite doing well in league tables.
Mark Twomey, chair of governors, said the need to employ a Catholic had "reduced the pool available by at least 90 per cent".
Heads' unions said the research by Professor John Howson, of Education Data Surveys, suggested the problems will only worsen in coming years as more senior staff reach retirement age.
The findings are based on a survey of more than 3,000 head, deputy head and assistant head posts advertised in England and Wales between August 2004 and July this year. They show that for the fourth year in succession London has the greatest problems with headteacher recruitment.
The high cost of housing has often been cited as a reason for the shortage of senior as well as junior staff in the capital. But the report suggested that, if anything, housing was less of a worry for London heads because the property market had been flat in the past year and teachers have been able to benefit from the Government's key worker loan scheme.
But another reason may be that heads can now get wages outside London which were once only available in the capital, such as salaries of pound;100,000 for running secondary schools. The report said that the academies programme had "undoubtedly increased the salary expectations in the secondary sector".
Professor Howson said he was disappointed that the annual report, which he has produced since 1995, showed that few ethnic-minority teachers had been appointed to the most senior posts.
Only 1.5 per cent of heads appointed in the past year were not white.
Professor Howson said this was "disturbingly low" and called on the National College for School Leadership to investigate.
There was better news for female teachers. Women accounted for 72 per cent of new primary heads, up from 68 per cent last year and 44 per cent of first-time secondary heads, up from 36 per cent. "They may finally be breaking through the 'glass ceiling'," Professor Howson said.