The trend towards increasing numbers of pupils entering GCSE maths early is damaging children's ability to do well in the subject, says the Advisory Committee on Mathematics Education (ACME) in England.
Last year more than one in 10 students sat GCSE maths aged 15 or younger, compared with just 5 per cent in 2008.
Now ACME has called on the Westminster Government to make it clear that pupils should continue to study maths until the end of Year 11, even when they already have a GCSE in the subject.
Dame Julia Higgins, chair of the advisory body, said: "We are seeing a worrying increase in the numbers of students being entered early for GCSE mathematics, to the detriment of almost all students.
"It's no longer a case of a careful selection of the brightest students being pushed through early - it's whole cohorts now, whether or not it's in their long-term interests as individuals.
"The pressure on schools to improve their standing in the league tables provides an incentive to act in the school's best interests rather than those of the individual students."
Dame Julia's organisation claims that some 25 per cent of schools enter students early for GCSE, but those attaining a grade C immediately stop studying mathematics, making it harder to return to the subject later on.
Andrew Hall, chief executive of exam body AQA, said his organisation was also concerned about the issue. "Early entry in itself isn't a bad thing - but it is crucial that decisions are made to suit the needs of the learner," he said.
"There is evidence that some learners don't perform as well as one would expect, had they been entered at 16. Our initial research shows that while early entry is right for some, there can be a negative impact for mid- range performers, as opposed to the highest performers."
A DfE spokesman said: "It's right that schools are free to make judgments about when their pupils are ready to take GCSEs. We're reforming league tables to get rid of any perverse incentives to enter young people for subjects which boost the school's position but don't benefit the individual student."