Private-school students will find it easier to win places at leading universities under a new grading system proposed for A-levels, TES Cymru can reveal.

Plans to divide the A grade into three, designed to help admissions tutors select the highest achievers, will result in far higher proportions of private pupils getting the top grades.

Universities will find it easier to defend the high percentage of applicants they take from independent schools, and might even take on higher numbers of private candidates.

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. The AQA board reached similar conclusions for its own A-levels last year.

The Welsh exam board, WJEC, has yet to carry out an analysis of its results. But it will introduce the new grading system at the same time as its English counterparts. And teaching unions fear the changes could have a particularly heavy impact in Wales, where there are few independent schools.

Anna Brychan, director of the National Association of Headteachers Cymru, believes that Welsh, state-educated pupils will be disadvantaged when competing against English pupils for university places.

She said: “If you’re offering wider provision, it needs more teaching and resources. Wales has lots of very small schools, and serious resource issues, so that would not be feasible for them.

“When our students are preparing for university, they could lose out dramatically to those going through the private sector.”

The University of Cambridge Local Examinations Syndicate, which runs the OCR board, analysed more than 50,000 A-level results of students in six subjects this summer. It introduced a hypothetical A-plus-plus grade for candidates with 560 marks out of 600 or more, and an A-plus grade at 520 marks out of 600.

In every subject, the proportion of pupils from private schools achieving A-plus-plus and A-plus was higher than the percentage 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.

Under the current system, private-school candidates in the six subjects are twice as likely to get A grades as their state counterparts. They would be four times more likely to achieve A-plus-plus.

Edexcel carried out similar research on its own A-levels this year. A spokesman said the analysis was not being released, but that it had reached similar conclusions.

Heledd Hayes, head of education for the National Union of Teachers Cymru, said: “Those in private schools are lucky to benefit from smaller class sizes. But it does not mean their ability is greater. This adds an extra layer to an already creaking system. It does nothing to ensure that pupils who will benefit most from a university education are getting it.”

But Amanda Wilkinson, of Higher Education Wales, which represents Welsh universities, doubts that Welsh pupils will be disadvantaged.

She said: “A range of factors need consideration. For example, the number of Welsh A-grades is higher than the national average, so the lack of private schools is less important. You can’t look at it in bland geographical terms.”

If accepted, the A-plus-plus plan could be introduced by 2008.