Hundreds of schools saw a large fall in the number of pupils scoring at least a C in GCSE English this year, heads have said, as they warned that the fiasco could be repeated in the future.
There are "strong grounds to be fearful" that the problems seen this summer could happen again next year, not just in English, but other subjects as well, the Association of School and College Leaders (ASCL) said.
In an open letter, the union said that a quarter of secondary schools, around 750 in total, saw at least a 10% drop in the number of pupils achieving at least a C in GCSE English this year, and around a fifth, 600 schools, saw at least a 15% drop.
It also warned that the scandal had "far reaching implications", and could affect major education reforms, such as the Government's plans to replace GCSEs with a new English Baccalaureate Certificate.
The letter, addressed to Graham Stuart, chair of the House of Commons education select committee, sets out the ASCL's concerns about the grading fiasco, and the latest findings of their investigation into what happened.
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.
An investigation by Ofqual, the exams regulator, 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.
In its letter, the ASCL calls for Ofqual to take urgent action to remedy the situation for affected students, and called on the committee to "publicly recognise the need for their results to be regraded". "As the weeks pass, the impact on the lives of those students becomes more and more difficult to reverse," the letter says.
It repeats calls for an independent investigation, saying, "what has happened this year has far reaching implications for the integrity of our examination system and the planning of future reforms, including those currently proposed by the Secretary of State.
"In addition, we believe there are strong grounds to be fearful of a repetition of this kind of problem in next year's GCSEs in English and other subjects."