Ofqual urged an exam board to alter its GCSE English grade boundaries just two weeks before the results were published.
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.
The row over the GCSE English results broke out last month after it emerged that grading boundaries for the subject were altered between January and June.
Angry headteachers claimed that exam boards raised grade boundaries in the subject halfway through the year amid fears that too many children were going to get a C.
John Townsley, a headteacher and former Ofqual board member said that Ms Stacey's position had become "untenable".
Ofqual today 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 MPs.
She is due to appear before the cross-party Commons education select committee this morning.
And last night Welsh education minister Leighton Andrews demanded that the English GCSE papers of hundreds of pupils in Wales were regraded. They are now expected to receive higher grades.
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.
Edexcel replied the next day, just two weeks before national GCSE results were published on August 23.
In it, the awarding body warns that the grade predictions cited by Ofqual, which are based on pupils' SATS results at age 11, "can only be, at best, an indicator of performance."
Other factors such as changes in the year group and the new, modular exams system also have to be taken into account.
Edexcel concludes: "We believe this to be compelling evidence that our award is a fair award and we do not believe a further revision of our grade boundaries is justified."
It adds that Ofqual still wants an alteration, that the board could move the minimum requirement for a C grade for one of the English units up to 65 marks out of 96 - 10 marks higher than in January when it stood at 55.
Ofqual replied on August 9, with a letter from Mr Opposs that stated Edexcel is obliged to ensure its results are consistent with other awarding bodies, whose results were close enough to predictions.
"It is for you to decide how that is achieved," the letter says.
Mr Townsley, head of the Morley and Farnley academies in Leeds told the TES: "We can see, in the most certain terms possible, Ofqual applying immense pressure to the awarding body concerned in order to bring down the number of C grades awarded.
"Glenys Stacey's position is untenable and she should resign with immediate effect."
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."
This was the job Parliament intended the independent regulator to do, he said.
"The correspondence with Edexcel was part of that process and was entirely proper. We will set this out in detail in our final report.
"The exam board made the final decision on the grade boundaries."
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."
Teachers have estimated that thousands of pupils were affected by the move, with concerns centring around those who were expected to get a C but ended up with a D.
Overall, 63.9% of GCSE English exams were awarded at least a C, a 1.5% drop on the year before.
Ofqual conducted an inquiry which concluded that January's GCSE English assessments were "graded generously" but the June boundaries were properly set and candidates' work properly graded.
The regulator insisted it would be inappropriate for either of the sets of exams to be regraded. Instead, students will be given an extra chance to resit the GCSE in November.