Recommendations made by the School Teachers' Review Body (STRB) and accepted by Education and Employment Secretary Gillian Shephard will force governing bodies to consider paying an allowance to an acting head or deputy within one month of the teacher taking on the extra workload and responsibilities.
Guidance from the Department for Education and Employment will stress the allowance should be paid from the time the individual takes on the absent person's authority and, where necessary, backdated.
The allowance should mean acting heads or deputies receive a total salary equivalent to at least the minimum point on the pay spine for the school group. But the stand-in may receive less than the person being covered where, for example, a head has previously been awarded further discretionary spine points.
Acting heads and deputies will also inherit the conditions of employment of their temporary post. So a classroom teacher who becomes acting deputy may have to work more than the maximum 1,265 hours a year set down in the School Teachers' Pay and Conditions Document.
The pools of acting heads that were run by some local authorities were drained soon after schools were given control of their own budgets.
With many schools facing difficulty filling headships, and the strains of the job encouraging more heads to retire on grounds of ill-health, the number of positions filled by acting heads is widely believed to have increased.
The obvious way for a school to cover for an absent headteacher or deputy head is to ask another member of staff to take on the individual's role. But while acting headships can be an ideal form of personal development, unions are anxious that ambitious teachers are not exploited by governors hoping to keep a tight rein on spending.
Mike Beard, assistant secretary for salaries at the National Association of Head Teachers, said schools' approaches varied considerably. "Some schools will not pay an acting allowance until someone has been in post for 12 weeks. Others pay it from day one," he said. "Some schools will backdate it and some won't."
Mr Beard said many governing bodies had continued the practice adopted by their local authorities before local management. He welcomed the STRB's recommendations, stressing the NAHT had always believed acting heads and deputies should be paid an enhanced salary from day one.
Kath Brooke, the Secondary Heads Association's salaries officer, said most acting heads and deputies were treated fairly but the guidelines would provide additional protection where it was needed. She added: "You may find the odd maverick school which has no policy on the matter."
Local authority leaders told the review body there was no reason to lay down strict guidelines as current procedures were working well. The STRB accepted the problem was not widespread, but argued that guidelines would help establish consistency while leaving individual governing bodies with some discretion.
Although SHA has never formally taken up the case of an acting head who was unfairly treated, it is sometimes forced to have a "quiet word" with governors. And a National Union of Teachers spokeswoman could not recall the union fighting on behalf of a member who had taken on the role of acting head or deputy without being properly rewarded.
Tony Meredith, the Association of Teachers and Lecturers policy officer, described the STRB's recommendations as a "tidying up" exercise. While most acting heads and deputies were fairly rewarded, he said, the steady disappearance of deputy posts meant teachers who were middle managers were taking on extra permanent responsibility without receiving more pay.
The STRB also recommended that the pay and conditions document should be amended so teachers who took periods of sick leave would usually gain experience points for the periods they were absent from school.
Governors will have the discretion not to award points - notably in cases where a teacher is away from school for a long period due to illness - but must always award them during absences for holiday and maternity leave.