Unpublished research by the AQA board suggests that the gap between independent school candidates and their peers might grow if an A-plus, or "supergrade" were introduced.
Universities find it increasingly difficult to differentiate between candidates achieving As, and some private-school heads say the figures back their case for the introduction of the new grade.
Others, though, want universities to be given access to a student's marks in each subject.
The AQA carried out the research on seven subjects, assessing how students would have performed if an A-plus grade had been available last year.
Students would need 560 points out of 600 to gain an A-plus, rather than the 480 currently needed for an A. In geography, 1.7 per cent of comprehensive students would have achieved an A-plus, compared to 6 per cent from the private sector. In physics, the figures were 1.6 per cent and 9 per cent. In other subjects the gap was smaller.
The findings were presented to the Headmasters' and Headmistresses'
Conference and Girls' Schools Association. An AQA spokesman said the research, commissioned by the board in response to the debate about how to differentiate between top candidates, related to only a small proportion of subjects offered.
Leading private-school headteachers have argued that the figures show their candidates are being discriminated against by top universities. However, private students are over-represented at Oxford and Cambridge universities when compared to the proportion of their candidates achieving three or more As at A-level.
A government inquiry into university admissions has suggested that schools make greater use of scholastic assessment tests to supplement predicted A-level grades in choosing candidates.
Ken Boston, chief executive of the Qualifications and Curriculum Authority, says universities should have access to a student's A-level marks by next year.