Taking the maths GCSE early or multiple times damages pupils' education in the subject, experts have warned.
As teenagers across the country anxiously await their results, mathematicians condemned the practice and blamed "a target-driven culture" for skewing behaviour in schools.
The comments, by the Advisory Committee on Mathematics Education (ACME), come just weeks after England's exams regulator Ofqual suggested that the importance of gaining at least a C grade in maths and English as well as the pressure of league tables are fuelling moves towards early and multiple entry.
Figures show that tens of thousands of pupils took papers for more than one maths GCSE last summer, while the numbers sitting key exams before age 16 has soared.
It is expected that these strategies will affect next week's GCSE results, with predictions that fewer teenagers will score decent grades in subjects like maths and science for the second year running.
Last year 58.4% of maths entries were awarded at least a C, down 0.4 percentage points on the year before, while 60.7% of science entries were awarded A*-C, down from 62.9% in 2011. There were also falls in physics, chemistry and biology.
Overall, 69.4% of all GCSE entries were awarded a C or higher, down from 69.8 the year before.
ACME committee member Richard Browne said: "Early and multiple entry for GCSE mathematics damages mathematical learning.
"A target-driven culture based on league tables has skewed behaviour in schools and encouraged multiple entry.
"The practice of early entry has a negative effect on most students' mathematical education and hinders their progression to a wide range of subjects post-16 and in higher education. Repeated resitting and multiple entry reduces the time spent developing student's mathematics skills and knowledge, as well as having significant extra cost for schools."