Pupils capable of obtaining only a C grade at A-level two decades ago could expect to be awarded an A grade today, according to a study of exam standards for the government’s statistics agency.

The research finds that on average A-level grades for pupils of the same ability improved by two grades between 1988 and 2006. In maths the rise was far higher — 3.5 grades.

The report, written for the Office for National Statistics by an academic at Durham University, will add to the row over “grade inflation” and whether exams have become easier.

It comes ahead of A-level results to be released next week that are expected to show further increases in the percentage of pupils gaining A grades in many subjects. Last year the overall proportion of As rose to 24.1% of all exams taken, double the figure in 1990.

Robert Coe, director of secondary projects at Durham University’s curriculum, evaluation and management centre, said his research had not precisely identified why grades had risen so steeply at A-level and, to a lesser extent, at GCSE, but in his opinion the most important factor might be changes to the exams themselves.

In 2002 A-levels were broken into separate modules, each with its own papers, rather than pupils having to sit “big bang” exams after two years. Individual modules can now also be retaken, meaning that results can be improved.

Despite the government’s faith in the strength of the A-level, measures are being taken to make it more difficult to win top grades. From next year there will be an A* grade above the present top mark. Longer essays and tougher questions are planned and curbs on the number of retakes are being considered.

The changes follow complaints from universities that so many pupils have been obtaining A grades that it is difficult to distinguish between applicants.

Coe’s paper is based on results from the thousands of pupils who each year take an aptitude exam called the test of developed abilities, which measures pupils’ skills in a range of subjects without testing curriculum knowledge. This provides a consistent measure of ability from year to year against which grades can be compared.

In GCSEs Coe found there was an increase of about a third of a grade between 1996 and 1998 for pupils of the same ability, and since 2004 a rise ofa further fifth of a grade.

David Laws, the Liberal Democrat education spokesman, said: “There has been a loss of confidence in the public examinations system and what the results tell us about standards.”

Ed Balls, the schools secretary, insisted: “With sustained improvements in the quality of teaching and increased investment in schools, we should expect exam results to get better.”

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