One in three students awarded the new A-star grade at A-level will come from private schools, a study by Britain's largest exam board has concluded.
The AQA findings show that the launch of the A-star in courses beginning next September could help cement private pupils' domination of admissions to top universities.
The figures were set out in AQA's evidence to the House of Commons' select committee inquiry into assessment, based on an analysis of pupil performance in 21 syllabuses.
At present, some 28 per cent of A grades go to privately educated sixth formers, double the sector's proportion of the overall entry.
Were an A-star to be introduced, the proportion would rise to 33 per cent, said AQA. Some 18 per cent of A-stars would go to students at sixth-form and further education colleges, with 47 per cent awarded to state schools. Both figures are lower than the relative proportions of entries from these sectors.
The board said: "There is a challenge here for the maintained sector, but these are only average figures. The reality is that there is a range of achievement in all school types, with many maintained schools more than matching their independent colleagues."
The AQA findings follow those from the Sutton Trust last week that 100 elite schools and colleges 80 per cent fee-paying accounted for a third of Oxbridge admissions.
In other evidence submitted to the committee, the QCA said 64 per cent of key stage 2 teachers believed teacher assessment was more accurate than test results. At KS3, 37 per cent said teacher assessment was more accurate, with a further 41 per cent saying it was as accurate.
Also giving evidence to the committee, the National Foundation for Educational Research, which produces current national tests, cast doubt over a new form of testing being piloted by the Government in nearly 500 schools this term.
The trial involves KS2 and KS3 pupils taking a test when their teacher believes them to be ready.
But the foundation said the new tests "could not support teaching in any direct way", since they would provide little formative information on improving pupils' performance.
A maths teacher gave the committee a vivid insight into the focus on "borderline" GCSE pupils at one school in Wokingham, Berkshire.
She said: "There is massive pressure in my school to boost students to grade C, virtually by any means," she said.
Borderline pupils were given after-school classes, the teacher said, while more able students who had asked to attend were refused, with no alternative offered.