The Ofsted chief inspector is against radical reforms that would see less able pupils sit simpler qualifications or take papers later on, it has been reported.
Instead, the government should ensure that existing GCSEs are stretching for all students, according to Sir Michael Wilshaw.
In an interview with the Financial Times, he indicated that he is in favour of a single exam system for all teenagers.
"The examination system that we have has got to be ruthless and stretching all pupils," he said. He told the newspaper that it is possible to do this "within the existing system".
His comments are at odds with proposals being considered by the government to overhaul England's exams. Under plans leaked last month, GCSEs would be scrapped with pupils sitting "explicitly harder" O-level style exams in traditional academic subjects such as English, maths, history, modern languages and science.
It was suggested that less able pupils would take easier old-style CSE qualifications. Education Secretary Michael Gove later said that he would like to see all students sit the O-level style exams at some point in their school career, with some taking them later than at age 16.
Asked directly by the Financial Times if he would support either of these proposals, Sir Michael said that he did not. He appeared to suggest that pupils should be able to take an exam again if they did not get a good grade the first time.
In a speech last month, Mr Gove then appeared to suggest that pupils could sit exams at different stages. He said that over the next decade, he wants to see at least 80% of young people on course to get good passes in English, maths and science in rigorous exams.
A Department for Education spokesman said: "We will be announcing our plans for exam reform in due course.
"But the current system already means that a large proportion of pupils take a version of the GCSE which means they are barred from getting anything higher than a C grade. We urgently need to raise standards for all pupils."