Hundreds of schools saw a large fall in the numbers of pupils scoring at least a C in GCSE English this year, heads said on Friday, 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 numbers of pupils achieving a C or better 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 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 their letter, 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 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."
A separate document, outlining ASCL's investigations into the scandal, reveals the numbers of schools the union believes have been affected.
It says that the quarter of schools that saw at least a 10% drop in A*-C grades, and the fifth that saw at least a 15% drop are balanced by a slightly smaller number who saw rises of a similar level in results.
It adds that the affected schools include a "complete cross-section", including academies, rural and inner-city school and those rated outstanding by Ofsted.
ASCL general secretary Brian Lightman said he believed that Ofqual had not yet got to the bottom of what happened with this year's GCSE English grading.
"We are now well into the first term of the school year. Teachers need to know what grade boundaries are like, and how things are going to be set for next year.
"There is a real risk that unintended outcomes could occur again."
This year's GCSE English was a new qualification, which was first introduced for teaching two years ago, with the first awards given this summer.
Ministers' plans to replace GCSEs in the core subjects with a new exam called the English Baccalaureate Certificate were revealed last month.
The courses, which are currently being consulted on, would be first taught from September 2015, with the first exams taken in 2017.
Lightman warned that this qualification could be a "disaster waiting to happen" if it is not properly implemented.
He said that whatever the new qualification looks like, it needs to be properly planned and implemented.
ASCL's letter comes the day after it was revealed that around 45,000 students who took GCSE English in the summer will resit all or part of the qualification next month.
Ofqual said, following its investigation, that, while papers will not be re-graded, pupils will be given an extra chance to resit papers.