Double science GCSE was made harder for less able students and easier for the more able in the late 1990s, the qualifications watchdog has found.
Exam boards made changes in response to slimmed-down curriculum
Changes by exam boards - in response to curriculum changes - led to "less effective" assessment, the QCA said.
But the demands made on students by the year 2000 were "more appropriate".
The comments come in the QCA's latest crop of reports on whether standards have been maintained in GCSEs and A-levels. It says they have.
Its review panel said significant changes were made to GCSE science double award syllabuses between 1995 and 2000, in response to a reduction in the amount of material in the curriculum.
The problems had centred on a move from a three-tier double science exam to a two-tier version.
By 2000, the lower tier paper was "significantly more demanding" for the less able and, to a smaller extent, those of middle ability.
"In contrast, extending the range of grades available on the higher tier papers, from B to A* in 1995 to D to A* in 2000, resulted in fewer questions requiring higher order skills", the QCA said.
"Reviewers considered that these changes had resulted in a less effective assessment regime for both the least able and the most able candidates."
There were also significant changes in the coursework component - making it less demanding, particularly for more able candidates.
But again, the reviewers "considered the changes had been appropriate."
"In several cases, especially for the lower grades, a candidate's coursework compensated for a poor performance in the written components and was the major determinant for the award of a grade.
"This phenomenon was compounded by coursework that did not appear to merit the marks it had been awarded," the report said.
"Evidence from candidates' work suggested that the general quality of coursework tasks was disappointing."
A QCA spokesman said it was discussing the findings with the exam boards - AQA, OCR, Edexcel, WJEC and CCEA.
"In September 2003, new specifications were examined for the first time and this will be kept under close scrutiny."
Doubts about coursework were also raised in a QCA review of GCSE geography between 1996 and 2001.
The coursework mark made an "important contribution to the overall grade, particularly for foundation tier candidates", the QCA said.
"Much coursework appears to be 'highly managed' by teachers," it added.
INDEPENDENT COMMITTEE ON EXAM STANDARDS
Prof Barry McGaw, director for education, Organisation
for Economic Co-operation and Development (chair)
Prof Caroline Gipps, deputy vice-chancellor, Kingston
Robert Godber, former head teacher, Wath upon Dearne Comprehensive School, Rotherham
The reports reinforce critical comments about coursework made recently by the taskforce on 14 to 19 education being headed by Mike Tomlinson.
Overall, however, the QCA's chief executive, Ken Boston, concluded that its latest reports showed that "standards in England are consistent and have been held at a high level for many years".
An independent panel set up to review standards following the A-level grading furore in 2002 will report this autumn.
In 2001, a previous independent committee said there was no scientific way of being sure that the standard of A-levels had been maintained over the years.
But it also said there was a natural tendency for examiners to give marginal candidates the benefit of the doubt - which had contributed to the "grade inflation" improvement in students' results in recent years.