Legal action over this summer's GCSE English exams has moved a step closer with exams regulator Ofqual vowing to "rigorously defend" its decisions.
Their defence comes in a reply to a letter of legal challenge sent by an alliance of pupils, schools, councils and professional groups.
"We have responded to the pre-action letter and are rigorously defending our decisions," an Ofqual spokesman said.
"Our work to understand why some schools' results differed significantly from their expectations is continuing and we will report again shortly."
In their letter, which was also sent to the AQA and Edexcel exam boards, the alliance sets out plans to take action over decisions by the boards to increase the boundary for a grade C in GCSE English between January and June.
They also propose taking action against what they claim was a failure by Ofqual to address the situation.
They argue that those pupils who sat the exam in June have been treated with "conspicuous unfairness".
The letter, 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 alliance's letter said: "It is inconceivable that two cohorts of students enrolled for the same course in the same academic year, who have undertaken the same work and invested the same effort and who will be competing in future for the same opportunities, should be subjected to such radically different standards of assessment and award."
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.