The marks required for top grades in English dropped dramatically between 1997 and 2002. Higher-tier candidates needed 66 per cent, or 14 percentage points less, in 2002 for an A, and 19 points less for a C.
In maths, the mark required for an A dropped by eight percentage points to 57 per cent over the same period, while the mark required for a C dropped from 30 per cent to 21 per cent.
The analysis is based on results from AQA, the biggest exam board, but England's Qualifications and Curriculum Agency (QCA) believes it is representative of the other English boards and probably of the Welsh Joint Education Committee. Welsh schools use syllabuses from all four boards.
The QCA analysis shows students being given A-C grades for progressively lower marks over a five-year period. However, the scores required for the top grades began to rise in 2003 and 2004, to 61 per cent for an A in maths and 74 per cent for A in English. But they were still lower than in 1997.
Experts suggested this pattern might be linked to new syllabuses, or the setting of progressively harder papers, making it necessary to drop the grade boundaries.
But value-added data from Durham university suggests the exams did get easier. Researchers mapped maths and vocabulary test results for thousands of Year 10 students against their GCSE results, and found the grades of students with the same maths and vocabulary scores rose significantly in the late 1990s and then levelled off.
Mike Cresswell, director general of the AQA, said: "It is a truism of educational assessment around the world that nobody can predict perfectly how difficult a particular paper is going to be."