Head teachers are demanding that Education Secretary Michael Gove sets up an independent inquiry into this year's GCSE results.
The National Association of Head Teachers (NAHT) has written to Mr Gove and the exams regulator Ofqual, saying that it has been flooded by complaints about late grade changes in English GCSEs.
Unions claim around 4,000 pupils expecting a C grade pass in English were downgraded to a D – delivering a potentially fatal blow to their chances of taking up places to study A-levels.
Exam boards are bracing themselves for legal action and record numbers of appeals over grades from angry pupils and their schools.
But regulator Ofqual says it believes this year’s GCSE results are "right".
Surrey teacher Chris Edwards accused Michael Gove of 'betraying' students In his letter, NAHT general secretary Russell Hobby said the score needed to obtain a C in English appeared to have jumped by 10 marks between January and June.
He said: "It has also become apparent that the moderation of course work and other controlled assessment has been significantly and adversely changed.
"As a result, many pupils have not got the results they were predicted, leaving them with question marks over their future."
He added: "For a significant number of young people and their schools, moving the goalposts in this way will jeopardise not only places at sixth-form college and other arenas of post-16 education, but university applications and beyond as well as the schools' status with regard to the Government's own benchmark."
Chris Edwards, an English teacher from Bishop David Brown School in Surrey, has also written to Mr Gove to accuse him of "betraying" his students.
Michael Gove is under pressure to order an inquiry into English grades He read from his letter on Sky News, telling Mr Gove his pupils "can't understand why someone would want to play around with their futures in such a cruel way".
"You have not simply moved the goalposts," he added. "You have demolished them, sold off the playing fields and left the dreams of these youngsters in tatters."
The Association of School and College Leaders, which represents most secondary head teachers, is reported to be considering legal action over the late grading system on the grounds that it could hit ethnic minority pupils and disadvantaged youngsters hardest.
Its general secretary, Brian Lightman, told the Times Educational Supplement: "We suspect the very ad hoc decision that was made to lift the grade boundaries has disproportionately affected certain groups of students and so we think it is very worthwhile to examine this further and gather evidence.
"If we are advised that legal action is the right way to challenge this, then that is what we will do."
Ofqual has said: "We are confident that standards have been maintained and that the grades awarded are right."
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