Ofqual urged an exam board to alter its GCSE English grade boundaries just two weeks before the results were published, it has been revealed.
Leaked letters show the regulator wrote to Edexcel amid concerns that there would be a rise in C grades, calling on them to act quickly to produce results that were closer to predictions for the subject.
The board responded a day later saying it believed its proposed grade awards were "fair" and there was no justification for further changes.
As details of the letters were revealed, Ofqual chief regulator Glenys Stacey faced a call to resign. John Townsley, a headteacher and former Ofqual board member said that Ms Stacey's position had become "untenable".
Ofqual said that the letters to and from Edexcel were "entirely proper" and part of the regulator's work to ensure that standards are maintained, and comparable to previous years. The correspondence, seen by the Times Educational Supplement (TES) is the latest twist in the GCSE English fiasco, and comes as Ms Stacey is due to give evidence to the cross-party Commons education select committee.
According to the letters seen by the TES, Ofqual's director of standards, Dennis Opposs wrote to Edexcel on August 7 with concerns that the board was set to award results which would mean the proportion of pupils awarded a C grade would be eight percentage points higher than predicted.
It called on the board to act quickly to "produce outcomes that are much closer to the predictions". "This may require you to move grade boundary marks further than might normally be required," he wrote.
An Ofqual spokesman said it was the regulator's job to "make sure standards are right". "When setting out our comparable outcomes approach, we have 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."
An Edexcel spokeswoman said: "Where the grade boundaries were positioned for GCSE English was clearly a matter of extensive discussion this year between exam boards and the regulator.
"As this correspondence shows, Edexcel made certain reservations clear to Ofqual, in the interests of maintaining standards. Our final award, which we believe was fair to all learners, followed specific requests from Ofqual to help them to do that on a national basis across all exam boards."