The number of teachers appointed to assistant head positions in primary schools has more than doubled in just five years, new figures have revealed.
During the same period, between 2005 and 2009, assistant headships in secondary schools also increased significantly, by 21 per cent.
The dramatic shift sparked a row among teaching unions that has led to the exclusion of the NAHT from the social partnership group that advises the Government on teachers' working conditions.
While the number of assistant headships has soared by 45 per cent across primaries and secondaries, deputy headships fell nationally by 7 per cent.
The marked change to the make-up of school leadership teams has prompted fears that there will not be enough deputy heads willing to go for headship, exacerbating the looming leadership crisis.
Some unions are also concerned that schools are opting to increase the number of assistant heads because the positions are not covered by the same regulations as regular teaching contracts. This means more hours can be demanded of them.
The social partnership unions became embroiled in a major row about the rise of assistant headships before Christmas. With the exception of the NAHT, they want a stricter set of criteria to be introduced for when schools appoint teachers to the role. This would include assistant heads having "line management responsibility for a significant number of people".
Martin Freedman, head of pay and conditions at the Association of Teachers and Lecturers, said: "We are not saying anything untoward is happening, but because of the increase we need to make sure the system is operating properly."
But Mick Brookes, general secretary of the NAHT, said the criteria would create serious problems, especially for smaller primary schools.
"We feel it would make life far more difficult for primaries to ensure they have the leadership support required," he said.
Making a teacher an assistant head, rather than giving a teaching and learning responsibility payment, allows them to be used more flexibly and is better for both the teacher and the school, Mr Brookes added.
The NAHT has been suspended from the partnership until other members, including the Government, have decided whether to impose sanctions.
The union has a chequered history with the group. It was an original member but walked out in 2005, only to return in 2007.
The position of assistant headteacher was introduced in September 2000 as part of a reorganisation of school leadership groups. However, assistant headships do not have to be advertised nationally, unlike deputy head roles.
Initially, numbers grew in London and the South East, where schools find it more difficult to recruit staff. The role has now grown in popularity across the country.
John Dunford, general secretary of the Association of School and College Leaders, said: "There are much stronger and larger fields for assistant headship than for deputies. It is important that each school is given the freedom to determine its leadership structure."
'DEPUTY HEAD LEVELS ARE APPROACHING CRITICAL'
The increase in assistant headships risks creating a severe shortage of applicants for headteacher roles, an education recruitment expert has warned.
Professor John Howson, director of TES sister company Education Data Surveys, said: "Schools are rebalancing their leadership teams by having fewer deputies and more assistant heads.
"My major concern is if you cut the number of deputy posts, you will not have enough people coming through, especially in the primary sector."
Professor Howson suggested that deputy headships should be on a temporary contract for five years.
If deputies do not want to pursue headship after that time, they should become assistant heads again to avoid the problem of "bed-blocking", he said.
"The number of deputy heads is, if not yet critical, approaching a critical level," he said.
Assistant headships have also been awarded to some heads of department as a way to retain staff, Professor Howson added.
There are examples of assistant heads who have skipped the deputy role to take over as headteachers.
AN ASSISTANT'S TALE
Helen Ball joined St Anthony's School in Chichester, West Sussex, as a student teacher, rising to the position of assistant head three years ago.
One of three assistants, Ms Ball said the job had given her the experience and confidence to go for headship within the next four years.
"It has been a great opportunity to work with like-minded colleagues and learn from them," she said. Ms Ball led the school's bid for specialist status and developing its sports provision.
St Anthony's, a special school, does not employ a deputy head. Headteacher Bob Griffin, who did away with the position, said that having a number of assistants instead helped the school run more smoothly.
"They have all worked their way up through the school," he said. "Without a deputy, they work better as a team."