Just 3% of teenagers could score the highest grades in new GCSE English and maths exams, it has been revealed.
Under major reforms, only a fifth of pupils who would currently achieve at least an A grade will be awarded a "grade 9" - the top result available in the new system.
This could mean that fewer pupils achieve the highest mark in the future.
Revamped GCSEs in English language, English literature and maths are due to be introduced into schools in England next autumn, with the first exams taken in the summer of 2017.
England's exams regulator has confirmed the new grading system for the qualifications, which will see A*-G grades replaced with 1-9 - with 9 the highest result.
Broadly the same proportion of students will achieve a grade 4 and above as currently achieve a grade C and above, Ofqual said, while broadly the same proportion of teenagers who currently score at least an A will gain a grade 7.
The top 20% of those who score at least a 7 will be awarded a grade 9, the regulator announced.
For example, according to last year's national results 3.3% of English GCSE candidates were awarded an A* - this equates to around 24,127 students.
Under the new system, 2.8% of candidates would have been awarded a grade 9 in 2013 - around 20,472 candidates in total.
In English literature, 5.5% of exams gained an A* last summer, while 4.6% would have scored a grade 9 under the overhaul.
And in maths, 4.9% of last year's entries - about 37,248 in total got an A*, while 2.9% - about 22,045 - would have achieved a grade 9.
These figures indicate that fewer pupils could achieve the very best results in these subjects following the introduction of the new grading system.
Chief regulator Glenys Stacey said that there will be "anchor points" between both systems, to ensure it is possible to compare " the old with the new".
"We believe grade C/4 is the right anchor point," she said.
"It also means that for those that rely on these qualifications and use them, they will know, if they were looking for someone to achieve at least a C boundary mark in the past, whether they're deciding, for example if that child should be accepted for another course of study, or a job, that they can equate the 4 to the C," Ms Stacey said.
She added that there are also more results in the new structure, with three grades - 7,8,9 instead of A* and A, and 4,5,6 instead of B and C.
"There have been strong calls from users of GCSEs that there needs to be greater differentiation at the top end," Ms Stacey said.
The new grade 5 will equate to the top third of marks for a current C and the bottom third of marks for a B, Ofqual said.
It added that this means that a new 5 is higher in the new scale than a C result and is broadly in line with what evidence suggests is the average performance level of 16-year-olds in other top nations.
At the moment, schools are judged on the proportion of pupils scoring at least five Cs at GCSE including English and maths, and this is changing to take into account pupils' results across eight subjects, including the basics.
It is not yet known which new grade will be used as a benchmark instead of C. If the new grade 5 is used, this is likely to be seen as an attempt to raise the bar and improve standards.
New GCSEs are currently being accredited by Ofqual and Ms Stacey said there was "no reason not to be confident" that the syllabuses for English and maths will be with schools this term.
She also said that the regulator is still considering whether the "top 20%" rule should be used across all subjects.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL) said: "Students must not be disadvantaged by the change in grading. What is important is that Ofqual sets out very clearly to teachers and students what is needed to achieve a specific grade. This is not the same as simply describing what statistical proportion of pupils will achieve a grade.
"Employers need a clear message that if a student has achieved a particular grade, it means that they have a certain skill or knowledge level. There will be a large number of students who end up with both letter and numerical grades so it is important to have a benchmark that shows how the two relate.
"We do have concerns about limiting the proportion of students who can get the highest grade. If students achieve at a very high level that should be recognised. Where standards have improved, the results must reflect this. Schools must be fully briefed and provided with specimen papers and examples of questions in advance of implementation."