Official statistics prove that schools were right all along about last year’s GCSE English grading controversy, it has been claimed.

When thousands of pupils missed out on crucial C grades in September 2012. the exams regulator was taken to the High Court by schools, local authorities and teaching unions, arguing there had been a “statistical fix”.

But Ofqual insisted the grades were correct, and that “standards were maintained”. The judges agreed, ruling that a re-grading would have created “further unfairness” by pushing standards out of line with previous years.

Now though, it has been claimed that official figures show that standards were not “maintained”, but were significantly tougher and led to almost 18,000 fewer students gaining crucial A*-C grades.

David Blow, a data expert for the Association of School and College Leaders (ASCL), presented the statistics at a conference organised by the union last week.

“This shows that Ofqual and the exam boards made a mistake,” he told TES afterwards. “Overall the grades were harsher than they should have been.”

Mr Blow looked at Department for Education and Ofsted statistics which compare details of pupils’ performance in national primary tests with their GCSE results. They show that 69.5 per cent of these “matched candidates” achieved an A*-C grade in English or English language in 2012.

But Mr Blow said that if standards had remained constant with 2011 then tthe primary test data showed it should have been 73 per cent of candidates - with another 17,805 pupils achieving an all important A*-C grade.

“That is the actual outcome and you can see starkly that English did go down,” Mr Blow, head of the Ashcombe School in Dorking, Surrey, told the conference. “We were proved right in the end.”

He claimed the evidence, which arrived too late to be heard in the court case, also calls into question grades awarded in this year’s GCSEs. They had been graded using 2012 as a reference point and were therefore likely to be out of line with 2011 grades.

The regulator also said exam board data on “matched candidates” revealed that last year’s results were a little better than expected.

The new figures contradict findings from Ofqual which has said that when changes in the students taking English and language GCSEs were accounted for, the proportion achieving an A*-C grade last summer fell by just 0.3 of a percentage point.

This week Ofqual - and the AQA and Edexcel exam boards also involved in the court case - said they remained confident that the grades were correct and that any discrepancies were because Mr Blow was using different data.

The ASCL leadership also distanced itself from Mr Blow’s comments. Malcolm Trobe, ASCL deputy general secretary, said: “ASCL does not wish to reopen the GCSE English issue as we are keen to move forward and engage with the planning for the future of GCSEs and other qualifications.”

Mr Blow said his analysis indicated that last year represented a clear change in standards. When matched candidates were considered they showed the end of a trend - going back more than three decades - where English had been graded “more generously” than maths.

His analysis is supported by a DfE statistical release on last year’s results, published in January, which said: “For the first time since progress measures were introduced, the percentage of pupils making expected progress in mathematics between Key Stage 2 and GCSE is greater than the percentage making expected progress in English.”