Schools face increasing difficulties finding head teachers, with only 4% of teachers wanting to do the job within the next five years, a survey suggests.
Head teachers say their workload is too large
The General Teaching Council for England predicts that four out of 10 vacancies will be unfilled by 2011.
But the government said the proportion of empty head teacher posts was less than 1% and was "stable".
The GTC survey of 3,665 teaching staff also found 34% of heads wanted to retire within five years.
Meanwhile, of those teachers with 15 or more years' service, 64% envisaged staying in the same role for the next five years.
But 27% of assistant heads and deputies planned to move up the career ladder.
GTC chief executive Carol Adams said: "We would have a problem even if more teachers were actively planning to become head teachers, because there are simply fewer teachers in the 34 to 49 age group.
"This means that teachers will need to become heads earlier, or schools may need to share head teachers.
"But still only 4% of teachers are actively thinking about headship as a career move in the next five years."
Earlier this year, the National Association of Head Teachers published a survey which suggested that up to half a million pupils in England's schools were without permanent head teachers.
It blamed a heavy workload for putting off potential applicants.
However, following the GTC survey, Chris Keates, general secretary of the NASUWT teachers' union, said the evidence of a recruitment crisis was "at best tenuous".
She added: "Many of the solutions to head teachers' workload rest with governing bodies which have been less than diligent in applying to head teachers the contractual changes which are already in place and designed to address this."
A Department for Education and Skills spokesman said: "Head vacancies have remained low and stable for some years now.
"Less than 1% of schools at any one time will have a head vacancy and even then there will always be an acting or temporary head in place."
He added: "We recognise that leading a school is a rewarding but also challenging role and that is why we are paying heads in inner London up to a six-figure salary from this September."