Plans to divide the A grade into three, designed to help admissions tutors select the best students, will result in far higher proportions of private pupils getting the top grades, reflecting their high scores.
Universities will find it easier to defend the high percentage of applicants they take from independent schools, and might take on even more.
The findings, which cast a shadow over ministers' attempts to broaden access to university, are revealed today in a study of the OCR board's A-level results and a separate analysis by Edexcel. England's other board, AQA, reached similar conclusions for its own A-levels last year.
Under the current system, private-school pupils are twice as likely to get A grades as their state counterparts in six main subjects. They would be four times more likely to achieve the proposed new top grade of A-plus-plus Another study published this week suggests that private schools' success is a result of a greater proportion of privileged pupils, not evidence of better schooling.
The University of Cambridge Local Examinations Syndicate, which runs the OCR board, analysed more than 50,000 A-level results in six subjects this summer. It introduced a hypothetical A-plus-plus grade for at least 560 marks out of 600, and an A-plus grade at 520.
In every subject, the proportion of private-school pupils achieving A-plus-plus and A-plus was higher than the proportion who were awarded A grades this summer.
In history, 56 per cent of A grades went to independent school students but they would have got 70 per cent of the A-plus-plus grades. In English literature, the proportions rose from 66 to 79 per cent, in French, from 49 to 61 per cent and in maths, from 37 to 51 per cent.
An Edexcel spokesman said it had been asked by the Qualifications and Curriculum Authority to carry out similar research this year. The analysis would not be released, he said, but had reached similar conclusions.
Geoff Lucas, secretary of the Headmasters' and Headmistresses Council, said that under the present system many private pupils' achievements are not fully recognised and the new system would rectify this.
The proposal to split the A grade into three was put forward in the Tomlinson report on a diploma system to replace A-levels by 2014. One in five A-level entries achieved an A this year.
If accepted by ministers, the A-plus-plus plan could be launched by 2008, before the diploma, with harder material introduced in syllabuses at the same time to stretch the brightest students.
Before then, universities could be given students' grades on individual papers, or told how many marks out of 600 candidates scored. Both changes would probably favour private schools.
Broadening access to university has been one of Labour's central policies.
Under the new Office for Fair Access, from next year universities will have to show a commitment to admitting more working-class students in order to be able to charge pound;3,000 tuition fees.
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