Teachers have warned of a leadership crisis as research shows more than one in five headships advertised have not been filled.
With the new term starting, teachers predict schools will be forced to share heads, particularly in inner city areas.
Teaching unions say potential candidates are being put off by workload, pay and a "football manager" culture where failure to improve results means dismissal.
Research published by the National Association of Head Teachers today shows 28 per cent of primary head teacher posts advertised nationally between August 2004 and July this year remained unfilled.
At secondary level the figure was 20 per cent, while 22 per cent of special schools advertising for a new head failed to find anyone.
With up to 50 per cent of school leaders expected to retire within the next decade, the union warned the problem can only get worse.
"We are on the brink of a recruitment crisis," said Mick Brookes, general secretary of the NAHT.
"There are more and more schools that are headless and we have a huge number of heads who are going to be retiring over the next five years, something like 30 per cent."
The number of teachers applying for secondary headships dropped 2.5 per cent last year, according to the NAHT, continuing a downward trend of the last four years.
Mr Brookes claimed schools in tough areas, such as those in many parts of inner city Birmingham, faced particular problems recruiting heads.
He blamed league tables and Ofsted inspections for heaping too much pressure on those in top posts.
"More and more people are saying it is a very high-risk job," he said.
"Particularly when the Ofsted team comes round. If you are caught out or if it goes wrong it is a bit like being a football manager except you are paid about a tenth of what they are."
Mr Brookes added: "There is clearly a problem in recruiting to leadership posts in tough areas. Who wants to join a school that is continually slammed in the league tables?
"People haven't realised that in order to raise standards you have to raise morale. Continually putting schools at the bottom of the heap is not going to achieve that."
Mr Brookes claimed some heads were already being forced to run two schools and that trend was set to continue.
He also blamed the plethora of new initiatives from central Government for putting off potential candidates.
"t is an impossible job both for secondary and primary heads having to continually put on new initiatives that don't seem to be properly thought out or funded.
"I have spoken to a number of friends who are heads who say they don't want to go back this term."
His concerns were backed by the Secondary Heads Association.
Dr John Dunford, the organisation's general secretary, said: "The range of responsibilities, over-accountability and the unprecedented rate of education reform are all disincentives to good candidates to apply.
"Yet school leadership remains the most rewarding of jobs in the influence that it can have over the lives of young people. We must not discourage the best teachers from applying."
In Birmingham, 26 head teachers have retired in the last 18 months - 17 out of the city's 311 primaries, six from 76 secondaries and two from 27 special schools.
Education chiefs say plans are being made to cope with the looming recruitment timebomb, caused by post-war baby boomers approaching retirement age.