Headteachers said on Thursday the tens of thousands of students having to re-sit their GCSE English exam next month after a scandal which was not their fault are suffering a "gross injustice".
Around one in 14 students - more than 45,000 in total - who took the qualification in the summer have opted to re-take exams, according to figures obtained by the BBC.
One school leaders' union said the numbers were "higher than expected", and warned that offering students an extra chance to resit was not the answer to the ongoing GCSE English fiasco.
The figures came as an alliance of schools, pupils, professional bodies and councils announced they will submit a legal challenge over the debacle to the High Court in the next week.
Thousands of students received lower than expected results in GCSE English in the summer after grade boundaries were raised between the January and June exam sessions.
Following an investigation by the exams regulator Ofqual, students in England, Wales and Northern Ireland were offered the chance to resit all or part of the course.
But headteachers say pupils who were affected should not have to re-sit as the problems were not of their making, and are calling for this summer's GCSE English papers to be re-graded.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said: "We've said all along this is not the solution, because if the exams are graded in the same way as they were in the summer then students will still find their results are down.
"What this shows is that it is a gross injustice that this vast number of students are being subjected to go through a resit when the fact is this is not their fault."
He added that the numbers re-taking all, or part of the course, are "higher than expected, and shows the extent of the problem".
Figures from four exam boards, AQA, Edexcel, OCR and WJEC, show that over 45,000 candidates have opted to take part in the November resit.
Of these, the majority, around 32,000, are candidates with the AQA board. This is because AQA has a high number of GCSE English students, with 380,000 sitting the qualification in the summer.
Both OCR and Edexcel each have around 4,300 candidates taking part in the resit, and WJEC, the Welsh exam board, has around 4,700.
It is not currently known how this compares to resits in other years. Figures for the January exam session, when pupils usually take resits, normally include students taking resits and those who are sitting the exam for the first time.
Lightman said that some of those planning to re-sit "will have already missed out on opportunities", and that the results of the November exams will be "too late for them to start things this year".
Other students will have started new courses at a lower level because they did not get the result they needed in GCSE English in the summer.
On Wednesday, a spokesman for the education alliance calling for a re-grading of this summer's exam, said that, following a meeting of legal representatives, it was decided that a claim for a judicial review will be put forward.
"We have now thoroughly examined the case that we have and we are convinced of the merits of our case, and the expectation that we will have a success to get the outcome we want - which is a re-grade for students," he said.
"We will be putting our claim together and submitting it over the next week."
The announcement came almost a week after Ofqual responded to a pre-action letter sent by the alliance, vowing to "rigorously defend" its decisions over this summer's GCSE English results.
The row over the English exams broke out as national GCSE results were published in August.
Ofqual conducted an inquiry into the fiasco, which concluded that January's GCSE English assessments were "graded generously" but the June boundaries were properly set and candidates' work properly graded.
The regulator insisted it would be inappropriate for either of the sets of exams to be regraded. Instead, students will be given a chance to resit the GCSE in November.
In Wales, education minister Leighton Andrews ordered the WJEC exam board to regrade Welsh students' English papers.
As a result, last month nearly 2,400 pupils who took English with the exam board received better results, after a review of the marking system.