Universities which discriminate against private pupils to boost their state school intake could breach human rights law, claims a report.
Independent school pupils could be treated unfairly, a report says
Professor Alan Smithers, an education expert at Buckingham University, said it was "inherently unfair" to pick students based on school type.
It could also breach the Human Rights Act, which precludes discrimination.
But his study also found state pupils performed better at university when it came to improving on A-level grades.
Professor Smithers was commissioned by the Headmasters' and Headmistresses' Conference (HMC) and the Girls' Schools Association (GSA) to examine the issue of university admissions.
His report, released at the HMC's annual meeting in St Andrews, said: "There is no case for instituting formal procedures to secure the entry of maintained school pupils on lower entry qualifications.
"Indeed to do so in blanket fashion could well make university admissions less fair than they are now."
Professor Smithers said the implications of this had not been tested.
Last week, official figures showed universities such as Oxford and Cambridge had missed their "benchmark" targets for admitting a certain proportion of candidates from state schools.
Both universities fell about 20 percentage points short, though the targets had risen considerably - and unreasonably, they argue - due to changes in the way they were calculated.
Universities failing to meet benchmarks could face financial penalties from the new admissions watchdog, the Office for Fair Access.
It could remove their ability to charge tuition fees of up to £3,000 a year.
Philip Evans, from the HMC, said he did not see the need for benchmarks.
"Once you start to treat a candidate as a representative of a particular group you move into the danger of individual unfairness," he said.
Cynthia Hall, president of the GSA, said most universities did not have the time or money to interview candidates and get a complete picture of them.
"Instead, they are saying 'do it on a quota of independent to state schools'.
"We say this is fundamentally unfair and is not based on any proper research material."
But Prof Smithers' report says: "Students from independent schools do somewhat less well in terms of degree results relative to their entry qualifications than students from LEA (Local Education Authority) schools."
But it went on to say that this difference was "small" and "not consistent".