- also suggests that pupils on the CD borderline were hit much harder than their higher achieving classmates.
It used statistics from the AQA exam board, which had the largest number of GCSE English and English language entries and has generated most complaints from schools about unexpectedly low grades.
David Blow, the head of the Ashcombe School, Dorking, Surrey who carried out the analysis said media reports that around 10,000 of pupils had been hit by the grading changes, had underestimated the problem.
Mr Blow, a member of ASCL's data group, looked at what would have happened to pupils who sat exams in June if the January grade boundaries remained in place.
Hisanalysis of the distribution of grades and marks suggests that half the 133,906 candidates who received a D in English in June, may have got Cs if the boundaries were not moved.
"The numbers of pupils potentially involved could amount to many tens of thousands of candidates," Mr Blow said.
Brian Lightman, ASCL general secretary, said: "The more feedback we get the more serious this crisis has proved itself to be. There is enormous concern across the whole sector."
The analysis also found that pupils who scraped a C in an English language controlled assessment unit on understanding texts and creative writing in January would have needed to score four extra standardised marks to achieve the same grade in June.
However, a candidate who just scraped an A in January on the same unit would have needed no extra marks to achieve the same grade in June.
In the EnglishEnglish language higher level exam, a pupil who got the minimum C grade mark in January would have needed an extra six marks for a C in June. But someone scraping an A in January would only have had to have achieve two extra marks for the same grade.
Mr Lightman said the changes would have a "devastating effect" on pupils just missing out on a C.
"A C grade or above in GCSE English is the key to so many career paths," he said. "There is a massive issue here particularly for the government's social mobility agenda as many of these pupils are from areas of high deprivation."
An AQA spokeswomansaid: "Grade boundaries change between exam series by different amounts at different grades and this is a normal feature of the assessment process.
"This variation can occur, for example, as a result of the way students' performance spreads across the mark range. 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."
Read more on the GCSE results