The exams watchdog told markers to raise the boundaries for English GCSE grades against their wishes two weeks before the results were published, it emerged last night.
The revelation, which appears to contradict the findings of an initial investigation by the Office of Qualifications and Exams Regulation (Ofqual), led to calls for its chief executive, Glenys Stacey, to resign.
The demand, from headteacher John Townsley, a former member of the Ofqual board and a respected principal of two academies, comes on the eve of Ms Stacey's evidence to the Commons Education Select Committee, which is investigating the grade boundaries row.
In a series of letters between Ofqual and the exam board Edexcel, Ofqual insisted the boundaries be raised despite examiners saying they were "fair" just two weeks before the publication of results.
In her investigation into the controversy, Ms Stacey concluded that the eventual grade boundaries were correct and set by examiners "using their best professional judgement, taking into account all the evidence available to them".
However, the letters, obtained by the Times Educational Supplement, revealed that the examiners' professional judgement was overruled. The last-minute changes demanded by Ofqual led to about 65,000 teenagers failing to gain a C grade pass and missing out on sixth-form and college places or apprenticeships.
Headteachers have called for an independent inquiry into the affair.
Last night, Mr Townsley, executive principal of Morley and Farnley academies in Leeds, accused Ms Stacey of "bullying" the exam boards and called on her to resign.
"The content of these letters is startling and makes absolutely clear that Ofqual itself has been at the centre of this disgraceful episode," he added. "Glenys Stacey's position is untenable and she should resign with immediate effect."
Ofqual said: "We've made it clear that where exam boards propose results that differ significantly from expectations, we will challenge them and intervene where necessary to make sure standards are correct.
"The exam board made the final decision on the grade boundaries."Reuse content