The research, which showed state-school pupils were 20 per cent more likely to get a first-class degree, seems to strengthen the arguments in favour of positive discrimination towards applicants from state schools.
One of the report's authors, Dr Bob McNabb, of Cardiff Business School, said: "Kids who go to independent schools are more likely to get better A-level grades because of the resources that their schools are able to put into their education.
"When everyone arrives at university, they are suddenly on a level playing field. People, on average, from comprehensives appear to be very motivated and have something extra for a given set of A-levels which enables them to do well."
The research, by Cardiff and Aberdeen universities, looked at the progress of 60,000 students who graduated in 1992.
The study, co-written with Professor Peter Sloane of the University of Aberdeen, and Dr Sarmisha Pal at Cardiff Business School, looked at gender differences but threw up the state school findings as a by-product.
The research found that for pupils entering university with identical A-level grades, those from state schools would, on average, get better degrees than those from private schools.
Dr McNabb said: "This finding has implications of significance for admissions policies. The implication of the study is there is a case for taking state school kids with worse A-levels than private schoolchildren. But the difficulty is how you put it into practice. What sort of discount are they going to get?" Oxford and Cambridge universities currently draw about half their undergraduates from independent schools even though they comprise only 20 per cent of all sixth-formers. Cambridge has said it would like to increase the proportion of state-school pupils to 65 per cent of its annual intake.
The Headmasters' and Headmistresses' Conference says that 44 per cent of students gaining three or more straight A grades in 1997 came from the private sector.
HMC secretary, Vivian Anthony, said: "We would strongly oppose any attempt at discrimination of any kind. We feel decisions about places at university should be taken up on the basis of the quality of the candidates."