Thousands of A-level candidates, many of them the most talented, might have been unfairly marked down last summer, according to three senior examiners with the A-level board used by most of the leading independent schools.

Students could have missed out on university places, they say, because the normal, careful checking procedures were banned by the School Curriculum and Assessment Authority, leaving candidates open to “erratic marking”.

The Oxford and Cambridge Schools Examination Board’s chief examiner in English resigned last year, claiming that he could no longer guarantee that the marks were fair. Dr John Saunders is now mounting a campaign to highlight the issue and will be writing to 100 leading headteachers in the New Year.

The OCSEB’s 11 principal examiners in English found themselves suspended after a heated row over the issue and, as a result there are still no papers for next year’s OCSEB (now renamed the Oxford and Cambridge Examinations and Assessment Council) English exam.

According to Dr Saunders, some 10 per cent of candidates in some exams might have ended up with too low a grade. The problem affects students on syllabuses offering a choice of “modular” or “linear” routes, believed to total several hundred thousand.

SCAA decided last year that all these candidates must be marked according to “modular” rules. As a result, he said, students on the traditional courses found themselves denied a final overview from experienced examiners.

“Thousands and thousands of candidates could have been unfairly graded, ” said Mr Saunders. His case is that any candidate who just misses an A in the traditional exam is automatically reviewed. Similarly with a candidate who achieves three As and a C on the separate papers.

Very often, says Mr Saunders, the discrepancy is the fault of the markers. It affects the most talented pupils who may give unusual or unorthodox answers, particularly in a subject like English. But under the modular rules, he said, there is no re-assessment of unusual marks or “near misses”.

“Hundreds of thousands of candidates have been denied the right for moderation on the level that matters,” he said. “We reckon that up to 10 per cent of these have been unfairly graded. It is very unjust.”

A-level marking, he believes, could never be absolutely exact, with some experts believing that individual markers would differ by as much as 5 per cent even on science scripts. “I believe that SCAA was unaware of the statistical consequences of its decision.”

In an article in The TES today Brian Martin, another former principal examiner with the OCSEB and the head of English at Magdalen College School in Oxford, said: “About 95 per cent of OCSEB’s candidates were not assessed in the way they expected, nor in the way advertised, when they embarked on the syllabus.

“It is distinctly possible that thousands of candidates who took that A-level in 1996 were unfairly graded because of the imposition of the modular system. ”

Another former principal examiner, who did not want to be named, said: “The point is that there must be many schools who expected a holistic awarding procedure. Their candidates did not get that.”

The principal examiners’ claims were denied by the University of Cambridge Local Examinations Syndicate, which now controls the OCSEB. “They are a gross misrepresentation of the position,” said Dr Ron McLone, director of UK examinations for UCLES. “The code of practice allows all examination boards to put safeguards into place for English just as in any other subject.”

The “overall” view advocated by Dr Saunders, he said, is is impossible for exams as complex as A-level. No individual examiner would be able to cope.