A-level marks are being "artificially" altered by exam boards to ensure that the expected proportion of candidates gain the new A* grade in a practice that heads fear will lead to pupils missing out on university places.
The Headmasters' and Headmistresses' Conference (HMC) has written to Ofqual, the exams regulator, protesting that the measure could be "unfair to individual candidates", that it falls outside the watchdog's code of practice and could leave exam boards open to appeals.
The watchdog says the practice is necessary to ensure consistency between exam boards and confirmed that it had taken place in a some A-levels, with results due to be published next week.
Geoff Lucas, HMC secretary, said: "If there was a student who needed an A* for a university place and they only got an A because the regulator felt the need to ensure consistency between awarding bodies that is going to be quite difficult to stomach."
He is concerned that Ofqual may have underestimated the number of A* grades likely to be awarded by not factoring in the extra motivation it will have sparked in pupils.
The HMC, which represents leading independent schools, claims Ofqual only chose to reveal the practice last week in a manner which it describes as "cryptic at best".
The watchdog has refused to say how many times it had been used.
The news came as a leading academic warned that the A* was a statistically less reliable measure of performance than other grades.
Ofqual's revelation will trigger uncomfortable memories of the grading scandal that engulfed Government and the entire exams system in 2002.
Back then the A-level system was also changing and a desire from the regulator to ensure consistency led to last-minute grade changes and a scandal that helped to trigger the resignation of an education secretary.
UCAS says 13 universities are using A* grades in their admissions this year, with some 6,000 offers made requiring at least one A*. Three Cambridge University hopefuls need three A*s.
Working out the number of successful A* candidates should be straightforward. The rules say they must achieve A grade standards and at least 90 per cent in their A2 papers.
But Ofqual also calculated guides to what proportion of pupils should gain A*s in each subject, based on last year's results, to ensure all exam boards worked to the same standard.
Controversially, where the proportion of A*s fall significantly outside these guidelines it has agreed that exam boards can deflate or inflate the value of certain marks when calculating final percentage scores.
This would allow them to award the expected number of A* grades and fulfil the 90 per cent rule, without distorting any other grades.
Mr Lucas said the practice could amount to an "artificial" devaluation of a pupil's performance, leading them to miss out on an A* grade and potentially a university place.
Ofqual chief executive Isabel Nisbet said the practice would only have been used in "exceptional" circumstances. If a board could provide a satisfactory explanation for the number of A*s falling outside guidelines, they could be allowed to stand.
"I am quite sure that we are doing everything a regulator can to ensure fairness for every candidate," she said.
- Original headline: Exam boards massage A* A-level marks, Ofqual admits