The exams regulator will begin its investigation into GCSE gradings this week following claims that thousands of students have been treated unfairly.
Ofqual 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 this week 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.
The Department for Education said it welcomed the move, while shadow education secretary Stephen Twigg urged the regulator's investigation to be comprehensive.
The Labour MP also called for an independent cross party parliamentary inquiry to find out what happened to cause the "fiasco".
In an open letter to Education Secretary Michael Gove and Ofqual, the National Association of Head Teachers suggested grade boundaries in English had been "significantly altered" during the year in response to fears the pass rate would rise again.
The union said it had been "inundated" with calls from schools about the situation, and urged Mr Gove to establish an independent inquiry.
The Association of School and College Leaders (ASCL) has insisted it is "not afraid" to take legal action against exam boards over the grade boundary reform.
General secretary Brian Lightman said yesterday: "ASCL warmly welcomes the announcement by Ofqual that they intend to investigate the GCSE English issue.
"It is essential that the injustice done to many thousands of young people is put right."
Some 69.4% of all GCSE exams were given at least a C grade - down 0.4 percentage points on last summer.
Mr Gove, who has promised to curb so-called "grade inflation", has rejected suggestions that there had been political interference in GCSE results.
He insisted that any changes in grades were the result of "independent judgments made by exam boards entirely free from any political pressure".
He said the reason some pupils have had poorer results than expected was partly down to a change in the system which meant their exams had been split into units and modules this year.
Asked about concerns that English papers were marked too harshly, Mr Gove told BBC News: "Yes, the number of As and A*s has fallen, but the number of Bs has increased, the number of Cs has fallen and the number of Ds has increased.
"That is a result of the independent judgments made by exam boards entirely free from any political pressure and I think that the various chief executives of the exam boards, and indeed the chief executive of the regulator Ofqual, have made it clear that these decisions have been made because the exam boards and the regulator have sought to ensure this year, as every year, that exam results are comparable over time so that we can all have confidence in the examination system."