Ofqual, the qualifications watchdog, revealed that the number of teenagers gaining elite grades was likely to drop sharply.
The existing eight-point grading system will be scrapped in favour of a nine-point numerical structure to coincide with the introduction of new GCSE exams in 2017.
For the first time, GCSE papers in England will be graded from 1 to 9 – with 9 as the top mark – in place of traditional A* to G grades.
Ofqual said the move was in response to calls for greater differentiation at the top end.
Chief regulator Glenys Stacey said the approach to grading would draw heavily, in the first year, on 'statistical evidence to make sure that there are clear 'anchor points' from the old system to the new'.
She added: "This will make sure that the year group of students are not disadvantaged, or advantaged, because of the introduction of the new qualifications, and will provide some certainty about what to expect at this time of significant change."
Schools will begin teaching the new GCSEs in English, English Literature and maths in September 2015. Under the plans, roughly the same proportion of students as have achieved a Grade C will be awarded a new Grade 4.
Ms Stacey said the regulator was 'hard-wiring the Grade C boundary to the Grade 4 boundary'.
She said: "It's very important we have at least one anchor point between the old and the new."
Brian Lightman, general secretary of the Association of School and College Leaders, said: "Students must not be disadvantaged by the change in grading.
"Harder exams in themselves do not lead to higher standards. Excellent teaching and clear leadership are what enable students to achieve more.
"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."
What do you think about all these changes?