Independent school pupils will benefit disproportionately from the new A* A-level grade and there will be huge variations between subjects in the percentage of candidates achieving the elite rating, statistical studies suggest.
A study of last year's A-level results by exam regulator Ofqual suggests the proportions achieving A*s will be as high as 45 per cent in Japanese and 41 per cent in Russian, but as low as 1 per cent in ICT.
Andrew Grant, chair of the Headmasters' and Headmistreses' Conferences (HMC), which represents the heads of top private schools, said he was not surprised by the Ofqual findings, which indicate the top grade will be much more prevalent among more traditional A-levels compared with relative newcomers such as media studies.
"Our own modelling suggests a higher proportion of A*s will be secured by independent-sector students," said Mr Grant, headteacher of St Albans School.
The new A* was proposed to help top universities differentiate between increasing numbers of applicants with straight A grades.
Mr Grant warned there was a "clear and present danger" that universities using the new grade to decide between applicants could find it conflicted with the push for them to broaden access.
In 2008, the National Council for Educational Excellence, a committee of education and business leaders appointed by the then Labour government, recommended that the A* grade should bed in for a "few years" before universities used it in selection. But Cambridge and several other elite universities are to make the new grade a condition of entry for some courses this summer.
John Dunford, Association of School and College Leaders general secretary, said: "If the A* grade benefits those who are crammed for A-levels or are taught by experienced teachers in small classes, it will prevent students with high potential but without those advantages from gaining the places they deserve in selective universities."
A-level courses in languages which attract only hundreds of entries are likely to have the highest proportions of A* candidates.
But there are also huge differences within the high-entry subjects, according to Ofqual.
But Ofqual chair Kathleen Tattersall said: "It does not mean that some subjects are easier or harder than others; rather it highlights difference in the cohorts taking those subjects."
Ofqual stresses that its study does not take into account other changes being introduced to A-levels this summer. These include the reduction of the number of modules in most courses from six to four and the introduction of extra "stretch and challenge" to final-year A2 exams.
Ofqual has revealed that exam boards are preparing for the possibility of a big increase in the number of results being queried, with extra examiners and processing and customer service staff on standby.
Cambridge Assessment, parent company of the OCR board, has expressed concern about the regulator's attempts to ensure that standards remain constant.
Last week, it said Ofqual's call for greater reliance on statistical evidence from previous years when deciding where grade boundaries lie could lead to pupils receiving the "wrong" grades.
Original paper headline: New A* grade could veer from 45% to 1% according to subject