State pupils might be left with just the basics of reading, writing and arithmetic, while those in the independent sector continued to take traditional academic courses in English and maths.
Geoff Lucas, secretary of the Headmasters' and Headmistresses' Conference, issued the warning in today's TES as a survey by the leading private schools uncovered serious reservations about the Tomlinson review of 14 to 19 qualifications.
Many of the 180 responses from heads highlighted the need for more clarity about the proposals. More than three-quarters wanted more detail on how GCSE and A-level exams would evolve into the diploma system.
The survey by the HMC, the Girls' Schools Association, the Society of Heads of Independent Schools and the Independent Schools Association, comes as Mike Tomlinson's working group prepares to publish its final report on the diploma.
The group is recommending that, in order to qualify for a diploma at 16 or 18, all pupils study a "core" of communication, functional maths and information and communications technology.
But Mr Lucas said that there was a real danger that "the minimum becomes the norm", with many students abandoning traditional maths and English courses because they would not need these for the overall qualification.
This, he suggested, would occur only in state schools. In private schools, pupils would typically be expected to continue with GCSE-type courses in English, maths, a science, a language, a humanities or arts subject, plus an ICT course.
Mr Lucas said: "One of the unintended outcomes of Tomlinson may therefore be a widening of the gap between the maintained and independent sectors.
This would be both highly divisive and regrettable."
Mr Tomlinson has argued that he does not expect large numbers of pupils to abandon GCSE-equivalent English and maths exams, because they will still be required to study the subjects under the national curriculum.
But the criticism will be worrying for ministers who have made tackling disaffection and improving England's embarrassingly low post-16 staying-on rates one of the review's priorities.
But Mr Tomlinson said that Mr Lucas's response was "flawed on many counts".
"The assumption is that maintained schools won't take the same line (on the curriculum) as independent schools. What evidence does he have for that? And every pupil will continue to take maths and English until the end of key stage 4."
Mr Lucas is not the only voice warning of possible problems surrounding the proposed emphasis on the basics under Tomlinson. In July, the TES revealed almost universal opposition among maths teachers attending a meeting to discuss the plans. English teachers are worried about teaching "communication" separately from English literature and language.
The Tomlinson inquiry will publish its final report, setting out detailed recommendations for ministers, next month.