Pupils are encouraged to sit GCSEs early by schools eager to boost their position in league tables - but they are performing worse than students who stick to the original time-scale, statistics show.
Schools minister Nick Gibb has voiced concern that a huge rise in early takers is a symptom of schools urging pupils to sit exams early so they can register a C-grade or better to improve their position in league tables.
He said: "I am extremely alarmed by the enormous growth in early entry for maths and English GCSEs over the last few years, and in particular by the relatively poor performance of those taking their exams at the end of Year 10.
"For the very brightest pupils there may be a case for early entry and of course these are matters for the judgment of teachers. That judgment must be informed, however, by the evidence of what is in the best interest of the pupil; not the best interest of the school's league table position."
In 2007 there were 67,000 early entries in English and maths GCSEs, equating to 2% and 5% of pupils entering the subjects respectively.
By 2010, however, the number had rocketed to 326,000, meaning that about a quarter of pupils in both subjects entered early.
Twenty-nine per cent of 2010 early entrants achieved an A*, A or B in maths GCSE, compared to 41% of end-of-course entrants. Thirty per cent of early entrants achieved an A*, A or B in English GCSE, compared to 45% of end-of-course entrants.
These figures were reflected in the findings that higher attaining state schools are less likely to enter pupils early than lower attaining schools.
The research also shows that pupils who achieved an A*-C grade were less likely to retake to achieve a higher grade.
A total of 98% of early-sitting D-grade pupils resat the exam, but just 63% of B-grade pupils did the same.