The exams regulator will begin an investigation into GCSE gradings after claims thousands of students have been treated unfairly.
Ofqual has admitted there were "questions about how grade boundaries were set in a very small number of units across the year".
The urgent review comes amid threats of legal action from local authorities and teachers.
It was revealed on Thursday that the proportion of GCSEs, taken by pupils in England, Wales and Northern Ireland, awarded an A*-C grade had fallen for the first time in 24 years.
In a letter to the National Association of Head Teachers, Ofqual chief regulator Glenys Stacey wrote: "We recognise the continuing concerns among students, parents and teachers about this year's GCSE English results.
"We will look closely at how the results were arrived at.
"We will do this quickly, but thoroughly, so that we ensure confidence is maintained in our examinations system."
Ms Stacey said she expected Ofqual to gather evidence in the coming days before meeting awarding bodies to discuss its findings.