Pupils from top state schools could be discriminated against by universities trying to widen participation, the president of a group representing private girls' schools has warned.
Pauline Davies of the Girls' Schools Association, fears that pupils from leading secondaries, both state and independent, could be disadvantaged under a scheme being piloted by some universities.
She said the scheme involves pupils being able to "demonstrate exceptional performance in the context of their schools".
This means that a student leaving a high-achieving school would have to show he or she had performed better, and had the same or greater potential, as someone with the same or similar grades who had attended a lower-performing school.
"This is a ridiculous way to widen participation," said Mrs Davies.
"Parents are being given themessage that sending their children to a good school will reduce their chances of winning a university place.
"Are we as a country going to say to bright teenagers and their parents that the only way they can be sure to get university applications taken seriously is to go to the worst schools?"
Bristol university, which has denied accusations of a bias against applicants from private schools, is piloting the scheme this year.
Its admissions criteria state that it will take "a number of factors into consideration, including educational and social context".
Mrs Davies, who is a member of the Schwartz Steering Group, set up by Charles Clarke, the Education Secretary, to assess university admissions, added that students should be selected on ability.This is best measured by performance at A-level and not on which school a candidate attends, she said.
Mrs Davies will sound her warnings about the scheme in her presidential speech at the GSA's annual conference which begins in St Andrews, Scotland, on Monday.
She will also tell the conference that GCSEs are the best way to assess 16-year-olds and that they must not be abandoned.
Some private schools have in recent weeks backed the creation of an international GCSE, which is exam-based and similar to O-levels, while many heads in the private sector would prefer GCSEs to be scrapped altogether.
But Mrs Davies will say that GCSEs are the best way of "giving pupils recognition of their achievements in subjects they may no longer pursue post-16".
However, she will add that coursework and external assessment needed to be cut in volume.
"It's no use students doing coursework which overlaps from subject to subject and does not develop their skills any more, while taking up time that could be better spent on learning about the subject," she said.
Her comments will be heard by Mike Tomlinson, the former chief inspector of schools and the man charged with heading a review of the examination system, who will also be speaking at the conference.
The next stage of his group's recommendations will be published in the New Year.
Mrs Davies added: "We will use the conference to express our concerns to Mr Tomlinson."