The examinations regulator Ofqual is to "look closely" at concerns over grade changes to English GCSEs.
In a letter to the National Association of Head Teachers, regulator Glenys Stacey said there were "questions" over how grade bands were set.
The government is facing a possible legal challenge over grading reforms which appear to have denied thousands in England a C grade in the core exam.
The Department for Education (DfE) said it welcomed Ofqual's decision.
The proportion of GCSEs - taken by pupils in England, Wales and Northern Ireland - awarded an A*-C grade fell for the first time in 24 years when the results were released on Thursday.
In an open letter to Education Secretary Michael Gove and Ofqual on Friday, the NAHT suggested grade boundaries in English had been "significantly altered" during the year in response to suggestions the pass rate would rise again.
Ms Stacey said Ofqual would look at the detail of grade C boundary changes over the coming days, to "ensure confidence is maintained in our examinations system".
'Quickly, but thoroughly'
The chief regulator added: "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."
Ms Stacey said she expected to gather evidence over the next week and that Ofqual would then meet awarding bodies to discuss its findings.
Mr Gove has come under increasing pressure from teaching unions who say it is unfair that pupils sitting the exam in June were marked more harshly than pupils who took it in January.
The Association of School and College Leaders (ASCL) has said it is "not afraid" to take legal action against exam boards over the grade boundary reform.
General Secretary Brian Lightman said the ASCL, which represents most secondary head teachers, "warmly welcomes" the Ofqual announcement.
"It is essential that the injustice done to many thousands of young people is put right," he added.
Earlier the ASCL called on the education secretary to take "immediate and decisive action".
Education officials at Leeds City Council have announced they are also considering a "legal challenge".
A statement on the council's website said: "We do not feel this basic principle of fairness has been adhered to in this case and will be looking with colleagues nationally at the possibility of raising a legal challenge to ensure Ofqual and the government put this right."
The NAHT said it had been "inundated" with calls from schools about the situation. The Centre for Education and Employment Research, a think tank, says thousands of pupils could have been affected.
Kathryn James, director of policy for the NAHT said she was "delighted" with the quick response from Ofqual.
"I'm still absolutely clear though that we do need an independent inquiry into what has gone on this summer.
"I think we need to be absolutely clear and make sure that this doesn't happen again - that we don't have this fiasco year upon year," she added.
Labour have demanded a cross-party parliamentary inquiry into what they have branded the "GCSE fiasco".
Shadow education secretary Stephen Twigg said: "Michael Gove needs to take urgent action as pupils may lose out on places in college.
"If that means getting exam boards and Ofqual in a room and banging some heads together, he should do that. Pupils must not be treated unfairly."
He added that Mr Gove had argued in the past for ministers to have greater accountability when problems arise in the exams system, and that the education secretary must "apply the same standards" now the coalition is in government.
In an earlier statement defending the reforms, the DfE said: "Ofqual is the independent exams regulator. Its job is to make sure that standards are maintained over time and that students receive the grades that they deserve."
"That's why we have strengthened Ofqual's powers to make sure the system is robust and rigorous and to give the public real confidence in the results."