The "stagnating and falling" number of traditional science graduates has been masked by a growth in topics such as sports science, says the Royal Society.
The institution also said suggested increases in maths and biology degrees were "apparent rather than real".
It criticised the way figures were gathered, citing changes to how those doing combined subjects were counted.
The new figures suggest that the popularity of subjects such a sports science, forensic science and psychology is masking a drop in those taking first degrees in biology, said the Royal Society.
While there had been an increase in graduates taking subjects categorised by Hesa as the "biological sciences", in 2004/5 biology students accounted for just 17% of this group - down from 31% in 1994/5.
Psychology degrees represent 47% of this grouping, up from 33% ten years previously. Sports science graduates had also risen nine percentage points since 1994/5.
And the rise in computer science students accounted for a general increase in degrees being awarded in the sciences - up from 31% of all degrees in 1994/5 to 37% in 2004/5 - said the institution.
Discrepancies in figures on maths and biology degrees had also been uncovered, according to the report.
While Hesa's annual figures had suggested a rise in maths graduates of more than 35% in the 10 years from 1994/5, the re-analysis indicated this growth was just 7.4%. Likewise a cited increase in biology graduates of 12.8% was only 1.7% when counted differently.
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