Lawyers for an education alliance taking action over the GCSE English fiasco are due to meet to discuss their next move.
The meeting comes a week after Ofqual, England's exams regulator, responded to a pre-action letter sent by the alliance, vowing to "rigorously defend" its decisions over this summer's GCSE English results.
Ahead of the meeting, one union leader said it is "almost certain" to bring an unprecedented legal challenge closer.
Russell Hobby, general secretary of the the National Association of Head Teachers (NAHT), which is a member of the alliance, said there was nothing in Ofqual's response which had changed their opinion of the case.
The education alliance, made up of pupils, schools, councils and professional bodies, sent a pre-action letter to Ofqual as well as the AQA and Edexcel exam boards, three weeks ago.
The letter set out plans for legal action over decisions by the boards to increase the boundary for a grade C in GCSE English between January and June. It also proposed taking action against what they claim was a failure by Ofqual to address the situation.
Two weeks later, Ofqual responded, with a spokesman saying the regulator 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.
Speaking ahead of the meeting, Mr Hobby said: "We didn't see anything in Ofqual's response that changed our opinion of the case."
He added that the case was about Ofqual "losing control of the system and trying to keep it quiet while students prepared for exams in good faith." Mr Hobby said: "More schools and authorities are signing up to the action and tomorrow is almost certain to bring action closer - that's certainly NAHT's view."
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.