"Hard" A-level and degree subjects are becoming the preserve of independent school pupils, analysis suggests.
An article published by the Independent Schools Council said independent pupils were getting more A grades in "harder" subjects such as sciences and maths.
This was not because of better teaching but because they accounted for more and more of the entries in those subjects.
The Department for Education has denied some A-levels are harder than others.
Shifts
The ISC article said research by Prof Peter Tymms of the University of Durham "has provided strong evidence that some A-levels are harder than others".
That study looked at the A-level grades achieved by students who had scored a B grade in their GCSEs.
Those doing history, economics, geography, English language and literature, sociology and business studies went on to score a grade C on average.
In maths, computing, German, French, chemistry, physics and biology the average was a grade D.
The ISC article, by research officer Sam Freedman, said that art was among "the very easiest" subjects.
Art and communications and media studies, design and technology and business studies had seen significant increases in the number of entries by state school pupils between 2000 and 2004.
Among the "hardest" were chemistry, physics, maths, French and German - which had all seen decreases.
'Extremely worrying'
In the independent sector the trend had been largely in the opposite direction - although there were fewer entries for French and German there too.
"Maintained school pupils are increasingly taking (or being encouraged to take) easier A-levels," Mr Freedman said.
The figures might explain why many of the top universities were finding it hard to increase the number of students they took from state schools despite extensive access programmes and overall improvements in grades.
"It seems that university departments in science, maths and languages are increasingly dependent on independent school students for their survival."
It adds: "The figures are extremely worrying. If nothing is done to reverse these trends, whole subject areas (and the university places and careers that go with them) will, to an uncomfortable extent, become the preserve of those lucky enough to be educated in the independent sector."
Mr Freedman suggests the government should "take more seriously" offers of help from the independent sector.
So far this had tended to involve facilities such as playing fields.
Instead there could be "virtual academies" in sciences and languages so state school pupils could benefit from the experience of independent school teachers.
A spokesperson for the Department for Education and Skills rejected the idea that some A-levels were harder or easier than others.
"All A-levels have strict standards which have been set by the awarding bodies and monitored by the Qualifications and Curriculum Authority," he said.
"This standard is set as advanced, regardless of the nature of the subject."