Exam boards are facing caps on the fees they charge schools, with qualifications watchdog Ofqual warning that this decade's huge hike in costs cannot continue.
The 83 per cent increase in the exam fees paid by secondary schools between 200203 and 200809 "will not be sustainable", Isabel Nisbet writes in her annual chief regulator's report released this week.
The Ofqual chief executive used a TES interview to stress that the prospect of fee caps in the next year "isn't an empty threat" and that surprise raids on the boards were also a possibility.
Ms Nisbet's warning came as it emerged that ministers have written to Ofqual, urging the watchdog to use its "crucial" capping power "whenever it is necessary".
Brian Lightman, Association of School and College Leaders general secretary, said some schools were spending more than #163;100,000 a year on exam fees, before invigilation costs.
"We are very pleased this is being recognised as a major issue, particularly in the current economic climate," he said. "It is difficult to justify this astronomical level of expenditure on assessment when schools are struggling to spend that on learning resources."
But exam boards argue that they offer value for money and they need to be able to afford to innovate and offer less popular qualifications.
The bigger exam bills can be partly explained by modularisation, a larger number of re-sits, an increased volume and variety of qualifications taken, and more early entries.
But Ofqual also uncovered large rises in fees for individual exams. It calculated that the average fee for an English GCSE rose from #163;23.74 in 200607 to #163;27.41 in 201011 - a 16 per cent increase, nearly double the 8.5 per cent inflation over the same period (see box).
Ms Nisbet said schools could be more "savvy" and work together in consortia to negotiate better deals from the boards.
Exam board OCR said the main reason for rising costs was more people taking exams. Chief executive Mark Dawe said the board was non-profit and only maintained a small surplus. AQA also stressed it was non-profit, adding that it kept fees as low as possible and that a key factor in rising costs was re-sits.
The Ofqual report warns there is a "danger" that some schools chose qualifications perceived as easier or cheaper instead of "those of an appropriate quality" to climb league tables.
Ms Nisbet said it would be "logical" for ministers to give Ofqual the power to oversee the league table values given to qualifications. The watchdog also wants universities to give clearer and more consistent messages about entry requirements.
"Students are not clairvoyants," Ms Nisbet said. "They need clear information on which qualifications they need for their chosen university courses."
She revealed that South Korea had been added to Ofqual's study comparing history, chemistry, maths and English A-levels with their international equivalents.
Ms Nisbet was optimistic about Shanghai being included and said discussions were continuing about adding the rest of China. But she was unable to reveal any progress with India, the other major Asian economy that ministers wanted in the study.
Secondary school spending on exam fees rose by an inflation-busting 6.7 per cent - or #163;17.7 million - during 200809, Ofqual revealed earlier this year.
The annual total of #163;281 million meant that schools had endured an 83 per cent increase in fees to exam boards in only six years.
The watchdog calculated that fees for GCSEs in English, maths, science, art and design, French and history all rose by an average of 16 per cent between 200607 and 201011. Over the same period the average fee for maths A-level rose by more than 12 per cent.