The growing trend in schools entering pupils early for maths GCSEs risks damaging the subject and education as a whole, a leading committee of maths experts and academics has warned ministers.
Figures released last week showed that 83,179 GCSE maths entries - more than one in 10 of this summer's total - were from 15-year-olds or younger, two and a half times the number of two years ago.
The Advisory Committee on Mathematics Education (ACME) is concerned that the phenomenon is caused by schools putting the demands of meeting targets and climbing league tables before the mathematical understanding of their pupils.
The committee, set up to provide Government with expert advice on maths, believes that the practice can leave pupils with an "absolutely critical" damaging gap of one or two years without studying the subject.
It argues that the gap can undermine later A-level and university studies and damage education in science, technology, engineering, maths (STEM) and "many other subjects".
In a letter to Education Secretary Michael Gove, ACME chair Professor Dame Julia Higgins, writes: "Learners gaining an early qualification is not necessarily a sign of success.
"Our concern is that the achievement of the C grade is overriding all other educational objectives, and as such the mathematical understanding of learners is secondary to targets and league tables."
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said: "I understand the concern that they feel league tables may be driving this, but there are many cases where students are entered early for valid reasons.
"We should be encouraging students to the very highest achievement across the board and one way of doing this can be early entry."
ASCL believes the sudden huge growth in early entries - only 32,908 under-16s were entered for maths GCSE in summer 2008 - has been partly prompted by the abolition of national tests for 14-year-olds.
The change makes it easy for schools to compress key stage 3 into two years, allowing a three-year key stage 4 with earlier GCSE entries.
ASCL thinks that while some schools may be entering less able pupils early so that they have time for resits, more will be hoping to allow the most able to progress further and faster.
But ACME believes it could have the opposite effect.
"We are concerned that students may stop learning mathematics altogether once a C grade has been obtained," Professor Higgins writes in her letter to Mr Gove. "As the exam entry may be in Year 10, or even earlier, this can lead to one or two years with no mathematics.
"This gap can prove absolutely critical in the future mathematical education of learners, undermining study at A-level and university, and in many other subjects beyond just STEM."
Roger Porkess, ACME member and chief executive of Mathematics in Education and Industry, said the committee had anecdotal evidence that "a lot" of early entries were driven by schools trying to improve their league table positions by maximising C grades in maths.
"I think the only thing that is going to stop this happening in the end is going to be changes to league tables," he said.
Nick Gibb, schools minister, has replied to ACME, noting its concerns about early entry and pledging to look at the impact of schools' actions when reforming GCSEs.