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Exam boards face a possible legal challenge from councils and colleges over grade changes to English GCSEs.

The Association of School and College Leaders (ASCL) says it may take legal action against exam boards over grading reforms which appear to have denied thousands a C grade in the core exam.

And Leeds City Council says it may also launch a similar challenge.

The Department for Education defended the reform.

In a statement, it said: "Ofqual is the independent exams regulator. Its job is to make sure that standards are maintained over time and that students receive the grades that they deserve."

For two decades, there has been a debate about "grade inflation", the statement added.

"That's why we have strengthened Ofqual's powers to make sure the system is robust and rigorous and to give the public real confidence in the results."

Education Secretary Michael Gove has come under mounting pressure from teaching unions who say it is unfair that pupils sitting the exam in June were marked more harshly than pupils who took it in January.

The proportion of GCSEs - taken by pupils in England, Wales and Northern Ireland - awarded an A*-C grade fell for the first time in 24 years.

Malcolm Trobe, of the ASCL, which represents most secondary head teachers, said it was currently gathering information on the situation at schools where there were large numbers of pupils on the boundary between a C and D grade.

"We're examining whether this is hitting any particular groups of young people that are covered by the equal opportunities legislation," he said.

"We are not afraid of taking legal action if that is the appropriate step."

Mr Trobe also called on the education secretary to take "immediate and decisive action".

"We do not feel this basic principle of fairness has been adhered to in this case and will be looking with colleagues nationally at the possibility of raising a legal challenge to ensure Ofqual and the government put this right."

AQA, the exam board with the highest market share for English, would not comment specifically on any potential legal challenge to the latest set of results, but did say it has a process in place whereby schools that are unhappy can contact it to discuss their concerns.

The NAHT union is demanding that Education Secretary Michael Gove sets up an independent inquiry into the changes, which one educational think tank says affected thousands of pupils.

Alan Smithers, of the Centre for Education and Employment Research, warned the marking reforms could have "drastic consequences" for schools and that "about 10,000 fewer" pupils have got a C grade in English this year compared with last year.

This would have a "very big effect", he said, because schools are judged on the percentage of their students who get five good GCSEs including English and Maths.

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

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 further education college or sixth form to study A-levels or other qualifications such as BTecs.

Charles Clarke, former education secretary, told the Today programme that the reforms to the English marking system had been "very badly and unfairly carried out".

But he also said that the improvement in exam results in recent years was down to rising standards in education.