Headteachers have said 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 on Thursday 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 confirmed it had responded to the letter and was "rigorously defending our decisions". He added: "Our work to understand why some schools' results differed significantly from their expectations is continuing and we will report again shortly."
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), 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."
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 regraded 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.