The exams watchdog Ofqual says it expects a "small drop" in grades in some of this year´s GCSE science results.
The Department for Education said it wanted "high-quality" GCSEs and it was for Ofqual to "appropriately" set standards in awarding qualifications. It is the first year a new version of GCSEs in biology, chemistry, physics, additional science and additional applied science will be awarded.
The watchdog says the syllabuses and exams for the subjects were changed because they were not tough enough.
"These GCSEs are designed to be more challenging, because the previous syllabuses did not adequately test the subject content and were not sufficiently demanding," the letter says.
Pupils in Wales and Northern Ireland also take GCSEs and for now, education ministers there have said they do not intend to move away from the existing GCSE and A-levels, but the growth of divisions in the system could bring confusion.
Other changes for England include the move to linear exams for GCSEs and A-levels, meaning that exams will be taken after two years of study rather than in stages or modules.
Next year will be the first time results are given for the new linear GCSEs, but the move to linear A-levels is planned to begin in September 2015, when teaching for those qualifications will begin. Teachers, students and parents are completely confused by all of these changes that are going on.
The letter also details the limiting of re-sits and move away from coursework.