Headteachers said today they are "hugely disappointed" that Ofqual and two of England's biggest exam boards have refused to take responsibility for the GCSE English fiasco.
As legal action over the issue moved a step closer, one union insisted it would not let the matter rest.
Ofqual responded yesterday to a pre-action letter, vowing to "rigorously defend" its decisions over this summer's GCSE English results.
The letter was sent to the regulator, as well as the Edexcel and AQA exam boards, two weeks ago by an alliance of pupils, schools, councils and professional groups,
It set out plans for an unprecedented legal challenge over decisions by the boards to increase the boundary for a grade C in GCSE English between January and June.
The alliance also proposed taking action against what they claim was a failure by Ofqual to address the situation.
An Ofqual spokesman yesterday confirmed it had responded to the letter and was "rigorously defending our decisions".
"Our work to understand why some schools' results differed significantly from their expectations is continuing and we will report again shortly," he added.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL) today said: "We are very disappointed that Ofqual and the awarding bodies refuse to take responsibility for their actions and to acknowledge that the way this year's exams were carried out meant that thousands of young people were treated unfairly.
"We will not let this rest.
"Ofqual says it has a duty to uphold standards, which is correct, but it should not do this at the expense of young people's futures.
"Our solicitors are looking over the response letters and we will be meeting soon with the other participants in the legal challenge to decide the best way forward."
The meeting is expected to take place next week.
In its pre-action letter, the alliance argued that pupils who took GCSE English in June have been treated with "conspicuous unfairness".
It called for June's papers to be re-graded in line with the January C grade boundaries.
If this does not happen, the alliance said it will seek a judicial review.
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 an extra chance to resit the GCSE in November.
In Wales, education minister Leighton Andrews ordered the WJEC exam board to re-grade 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.
Mr Lightman's comments come as emails show exam boards suggested to Ofqual that it was too closely involved with the grading fiasco to investigate.
According to emails and letters obtained through a Freedom of Information request, on August 25, the WJEC board sent an email to Ofqual which stated: "One question I think needs to be considered... is whether there are aspects of this investigation which need to be handled by someone independent of the regulators (as well as being independent of awarding organisations)."
In a separate email, an unnamed representative from Pearson, the parent company of Edexcel, said they had concerns that Ofqual is "in effect looking at whether grade boundaries are correct. The public will understandably see this as an investigation. I see this as surprising given Ofqual was party to their setting and we will say so."
In response, Ofqual said it will be clear in its report to say what it did, and what it asked exam boards to do.
Russell Hobby, general secretary of the National Association of Head Teachers (NAHT), today told the Times Educational Supplement (TES) that they want to see a full independent inquiry, even if results are re-graded.
"If we are to avoid making the same mistakes again we need someone independent to get to the bottom of what happened," he said.
An Ofqual spokesman said: "Ofqual acted quickly and thoroughly to look into the concerns around GCSE English raised by schools and teachers.
"While we have published an initial report, our work to explore why some schools got results that differed significantly from their expectations is ongoing and we will report again shortly.
"We are looking at all aspects of the issues around GCSE English, including the role of the exam boards, the role we played as regulator, the design of the qualifications and how things worked out on the ground in schools. Our focus is on getting to the bottom of what happened. Whether or not there is a need for an independent inquiry is not for us to determine."