Headteachers today claimed that tens of thousands of teenagers could have been adversely affected by grade boundary changes in GCSE English.
Pupils on the borderline of C and D grades were most likely to be hit, according to a new analysis by the Association of School and College Leaders (ASCL).
ASCL has been calling for this summer's English GCSE exams to be re-graded amid an ongoing row over changes to grade boundaries.
National GCSE results for England, Wales and Northern Ireland, published last week, revealed 69.4% of all GCSE exams were given at least a C grade - down 0.4 percentage points on last summer. It is the first time the A*-C pass rate has fallen in the 24-year history of GCSEs.
As the figures were published, angry headteachers claimed that exam boards had raised grade boundaries in English halfway through the year amid fears that too many children were going to get a C.
They have suggested that grade boundaries in this summer's English exams were substantially increased, with many reporting a drop in the number of children scoring a C or above in the key subject.
The new analysis, by David Blow, head of Ashcombe School in Dorking, and member of ASCL's data group, looked at the results pupils who took the exam in June would have obtained if the January grade boundaries had applied.
He said that his analysis of the distribution of marks and grades suggests that half of the 133,906 students who attained a D in English in June could have got a C grade if the boundaries had remained the same.
ASCL claimed their analysis used data from the AQA exam board, which it said had the largest numbers of entries for English and English language GCSE.
Mr Blow told the TES: "The numbers of pupils potentially involved could amount to many tens of thousands of candidates."
ASCL general secretary Brian Lightman said there was "enormous concern" across the education sector about the GCSE English results, adding: "The more feedback we get, the more serious this crisis has proved itself to be."
Earlier this week, the exams regulator Ofqual admitted that there are questions about how grade boundaries were set in a small number of English units over the year.
Ofqual met with education unions yesterday, as part of an investigation into the GCSE English results.
It is investigating the issue and is due to publish an initial report on Friday.
An AQA spokeswoman told the TES that grade boundaries can change between exam series, and that this is a normal part of the assessment process.
"This variation can occur, for example, as a result of the way students' performance spreads across the mark range," she said.
"So, at one level of performance candidates may have earned comparatively similar marks, and at another the marks might be quite spread out.
"In this case, examiners will take the spread of marks into account when they set the grade boundaries in order to reflect the appropriate standard."