More than 45,000 GCSE and A-level exam grades have been changed after schools made a record number of challenges about the results.
Exam watchdog Ofqual says the number of queries was up by 48 per cent to 450,500, calling into question the competence of the marking system.
School leaders have voiced concerns about the quality of marking this summer and feared that mistakes could mean that young people missed out on university places or being able to stay on to take A-levels.
Overall, about one in every 33 scripts marked this year resulted in an inquiry about marking or grades. Of these, 45,500 were corrected – up 15 per cent on the previous summer.
Brian Lightman, general secretary of the Association of School and College Leaders, told the BBC: "We're not surprised to hear that there has been a significant increase in requests for re-marks. Many schools have told us of a worrying number of results which simply did not reflect how well students should have done.
"Schools need to be able to trust the marks given to students. They need to know that examinations will be marked accurately, fairly and in a timely manner."
Mr Lightman said it remained unclear whether the increase in appeals is due to a 'fundamental weaknesses in marking'.
But he warned that the 'lack of confidence in the exam system which has been exacerbated by frequent and ad hoc changes to qualifications'.
The overall proportion of exam grades being changed remains low, but has increased for both A-levels and GCSEs.
This year, 0.52 of A-level grades have been changed.
Education Minister Nick Gibb said: "It is essential that students can be confident that their hard work will be accurately assessed and the exams they sit properly marked.
"Parents, pupils and schools must have faith in exam marking and we are working closely with Ofqual and the exam boards to ensure this is the case."