The marks required for top grades in English dropped dramatically between 1997 and 2002, according to the Government's qualifications agency.
Higher-tier candidates needed 14 fewer percentage points than in 2002 for an A, and 19 points fewer 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 figures have fuelled claims that the exam boards allowed standards to drop. Tim Collins, the Tory education spokesman, said they showed "a profoundly worrying trend".
"This does nothing to reassure teachers, parents and pupils - let alone admissions tutors and employers - that GCSEs are a reliable guide to effort and ability," he said.
The figures were placed in the House of Commons library after being compiled by the Qualifications and Curriculum Authority from marks required by the biggest exam board, the AQA. The authority said it believed they were representative of the picture across all English boards.
They show pupils being given A to C grades for progressively lower marks over a five-year period. By 2002, pupils needed 57 per cent to gain an A grade in maths, and 66 per cent in English.
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 69 per cent for an A in English.
But they were still lower than in 1997.
Experts suggested this pattern of change could be linked to syllabuses introduced in 1998 and in 2004. Alternatively, papers might have become progressively harder, making it necessary to drop the grade boundaries.
However, value-added data seems to support the suggestion that exams became easier. Researchers at Durham university have tested thousands of Year 10 pupils in maths and vocabulary each year for more than a decade. They mapped pupils' performance against their GCSE results and found that the exam grades of teenagers 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 the QCA's figures were based on syllabuses offered by its predecessor board, AEBSEG. Similar figures for an NEAB board English syllabus taken by more pupils did not show the same pattern.
"It is not sensible to interpret grade boundaries in terms of standards. It is a truism of educational assessment around the world that nobody can predict perfectly how difficult a particular paper is going to be.
Boundaries have to be able to move if we are going to maintain standards," he said.
But Roger Porkess, a former maths A-level examiner, said: "If you look at what students coming into sixth form can do, it hasn't improved. Algebra lets them down - that's always been the case, but it's even more so now."