Reforms made to GCSEs could result in large variations in grades, Ofqual has stated.
The exam watchdog said recent changes such as a return to end-of-course exams are likely to result in bigger than expected differences in grades.
Overhauls to the education system introduced by former education secretary Michael Gove prevent pupils from resitting modules during their course, meaning they are assessed at the end of their period of study.
As a result of the reforms, which mean only a student's first attempt at an exam is now included in performance tables, fewer pupils are being entered for exams early. About 300,000 fewer students were entered for GCSEs at age 15 compared with last year.
"These changes do not pull results universally in one direction or another, but together they are likely to affect the national picture to some extent," Glenys Stacey, Ofqual's chief regulator, told The Guardian.
Posted by Theo Foulds