Soaring numbers of primary schools are struggling to recruit headteachers, according to newly published research.
More than a third of primaries could not make an appointment after advertising between September 2006 and March this year, the study by Education Data Surveys found.
This was up from just over a quarter the previous year, according to the research, which was commissioned by two headteachers' unions.
Mick Brookes, general secretary of the National Association of Head Teachers, said the findings underlined the need for ministers to "review the pressure on school leaders".
"Good leadership is the key to continued academic success," he said.
"If the Government is to achieve its aim of raising standards it must throw its weight behind improving the attractiveness of leadership posts."
Of the schools that advertised for a new head during the period, 19% of secondaries and 35% of primaries failed to appoint.
The position had improved for secondary schools, which last year saw 22% fail to appoint after advertising for a new head.
But the situation was much worse for primaries, which saw 28% fail to appoint last year. Recruitment problems were also worse in special schools.
John Dunford, general secretary of the Association of School and College Leaders, said: "The improvement at secondary level is encouraging, but the greatly increased number of retirements from secondary headship between 2008 and 2011 will create a highly competitive situation between governing bodies, for good candidates."
