Universities could be in breach of human rights legislation if they discriminate against independent school pupils when awarding places.
A report commissioned by independent schools warned universities face being sued if they backed positive discrimination to meet government targets to recruit more state school students. Universities such as Exeter, Bristol and Cambridge have acknowledged they offer some state school students places on lower A-level grades but say it is as a result of decisions based on individuals rather than a blanket policy.
However, new and higher benchmarks for state school recruitment published by the government-backed Higher Education Statistics Agency last week have increased pressure on universities. They are now worried they will not be allowed to charge top-up fees if they fail to take action.
Yesterday's report, by Professor Alan Smithers, head of the Centre for Education and Employment Research at the University of Buckingham, argued it would be illegal to discriminate on grounds of gender and ethnicity - both of which have more impact on students' attainment. "It is difficult to see how positive discrimination on one characteristic can be against the law, while it can be advocated on another which has less of a connection with degree results," said Professor Smithers. Earlier research, he said, had shown students from ethnic minorities were twice as likely to go to university. "There is no suggestion, for obvious reasons, that one should seek to correct this imbalance through the admissions procedures."
His report said the universities were already taking a broader view of students' ability than just A-level results and offering state pupils lower entry qualifications. Research by the Higher Education Funding Council showed state school pupils with passes two grades lower at A-level were likely to achieve the same degrees as independent school pupils.
He said there was no need for discrimination, adding: "The system seems to have evolved in a way that is fair to the individual candidate."
Philip Evans, joint chairman of the Headmasters' and Headmistresses' Conference (HMC) and Girls' School Association universities committee, which commissioned the report, said schools would prefer to avoid legal action. "We think it is far more important to draw attention to this rather than let something happen and do something about it afterwards," he added.
However, independent school heads are incensed by new benchmarks announced last week which follow an earlier assurance from Margaret Hodge, the former higher education minister, that they would no longer take account of school background from 2007.
Professor Smithers' report also showed boys from Britain's top fee paying schools do less well at university. One of the theories put forward in the research is that students from high income homes are under less pressure to get a good degree due to financial security.
The report said: "For male students, but not female students, there was some association between the level of the fee charged by the independent school and degree performance. Another possible explanation is that the high performing direct grant schools which became independent in the 1970s tend to charge less than average fees."
Independent schools admitted yesterday that some were forcing pupils to drop exams subjects in order to boost their rankings in league tables.