Thousands of the brightest children in England are failing to achieve top grades at GCSE because of a growing trend towards entering pupils early for the examination, according to figures released by the education watchdog.
Sir Michael Wilshaw, Ofsted chief inspector, has warned the watchdog will be "critical" of schools which use early entry for GCSE where they are not "absolutely confident" that pupils will achieve their full potential.
His message comes after Ofsted statistics showed an "explosion" in early entries for maths and English at GCSE over the past six years with around a third of all pupils - more than 200,000 in each subject - now entered early for these exams.
But figures released by Ofsted have shown that for pupils who were the highest achievers at primary school - reaching level 5 in Sats exams - 12% fewer were awarded A grades in English and 11% fewer got A grades in maths in 2011 where they took the GCSE exam early, compared to those who were not entered early.
"We think early entry hurts the chances of the most able children getting the top grades of A*, A and B which they need to progress to A level and certainly to university," Sir Michael said. "We will be critical of schools using early entry except where they are absolutely confident that youngsters are reaching their full potential. By full potential we mean A* and A actually if they are bright youngsters."
Sir Michael added that even when pupils achieved those top grades early Ofsted expected them to continue studying the core subjects of English, maths and science into Year 11 rather than being diverted to other subjects.
He said around one in five, or 20% of children, who leave primary school with level 5 do not achieve the top grades at GCSE of A*, A and B as a result of a "combination" of factors including early entry at GCSE.
Other factors included low expectations in schools of pupils, a failure to track the progress of pupils sufficiently and what Sir Michael termed "the curse of mixed ability classes without mixed ability teaching."
A Department for Education spokesman said: "Candidates entered early often perform worse overall than those who do not, even after re-sits are taken into account.
"Some pupils are being entered before they are ready, and 'banking' a C grade, but their performance at Key Stage 2 suggests that if they had continued to study the subject and taken the GCSE at the end of Year 11, they could have achieved a top grade."
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