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Head teachers' leaders are demanding the education secretary sets up an independent inquiry into this year's GCSE results.

In a letter to Michael Gove and the exams regulator, the NAHT union said it had been inundated with calls about late grade changes in English GCSEs.

A new marks scheme appears to have unexpectedly denied many pupils the all important C grade in the core exam.

Regulator Ofqual says it believes this year's GCSE grades are "right".

But exam boards told reporters on Thursday that the C-D grade boundary had been raised by as much as 10 marks, or 10% in some cases, during the examining period.

Obtaining a C-grade in the core subject of English is crucial for pupils wishing to go on to a further education college or to study A-levels.

In his letter to England's education secretary Mr Gove and Ofqual chief Glenys Stacey, NAHT general secretary Russell Hobby said he knew from his members the GCSE results they had been anticipating had fallen short.

"As the picture emerged, it appeared that up and down the country English Language GCSE results in particular were down as a result of a different mark scheme being used in June."

This increased the score needed earlier in the year to obtain a grade C, by 10 marks, he said.

"It has also become apparent that the moderation of course work and other controlled assessment has been significantly and adversely changed," Mr Hobby said.

"As a result, many pupils have not got the results they were predicted, leaving them with question marks over their future."

Mr Hobby 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."

The Association of School and College Leaders, which represents most secondary head teachers, is reported to be considering legal action against exam boards 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."

The association is backing an investigation by the Academies Enterprise Trust, which runs 29 academies, asking schools across the country to send it anonymised information about the backgrounds of pupils who missed out because of the grade boundary decisions on the English GCSE.

Chris Edwards, an English teacher from Bishop David Brown School, in a deprived part of Surrey, has also written to Mr Gove, saying his many of his pupils "never stood a chance" when they sat their English exams.

"These are not privileged kids who were bright enough to get a high grade, but just couldn't be bothered to work.

"These are students who are learning English as a second, sometimes third language, who have attended every revision session provided."

An Ofqual spokesman said: "It is vital that the grades awarded for overall qualifications are right, and that standards are comparable year-on-year and across the exam boards.

"Where students take exams in different units spread across a couple of years, the exam boards have to consider all of the evidence available for each exam series and make decisions to make sure that the right qualification standards are achieved.

"We are confident that standards have been maintained and that the grades awarded are right."

Results for pupils who sat GCSEs in England, Wales and Northern Ireland were received on Thursday.