The country's top universities have been called on to come clean about an unofficial list or lists of "banned" A-level subjects that may have prevented tens of thousands of state school pupils getting on to degree courses.

Teachers suspect the Russell Group of universities – which includes Oxford and Cambridge – of rejecting outright pupils who take A-level subjects that appear on the unpublished lists.

The lists are said to contain subjects such as law, art and design, business studies, drama and theatre studies – non-traditional A-level subjects predominantly offered by comprehensives, rather than private schools.

The London School of Economics is thought to be the only top university to publish its own list of "non-preferred" subjects. Cambridge University did so until last year.

Teachers accused universities of putting comprehensive pupils at a disadvantage by refusing to publish their lists. Some claimed the lists were a filter that enabled the most prestigious universities to accept more private school pupils than state-educated ones.

Already private school pupils dominate entry to top universities and could do so more in future. A-level results published on Thursday showed that selective private schools continue to outperform comprehensives in terms of A and A* grades.

Private school pupils are three times more likely to score the highest grade than comprehensive pupils, achieving 30% of the total number of A* grades when their pupils accounted for just 14% of entries. Comprehensive schools achieve 30% of the A* grades on 43% of entries.

But even the brightest state school pupils, with a string of As and A*s, stand little chance of a place at a top university this year if they have taken one or more of the A-level subjects on the unofficial lists, the teachers said.

These students will join this year's unprecedented scramble for university places. An estimated 180,000 students are predicted to be turned away from every degree course starting this autumn because of record numbers of applications.

John Bangs, former head of education at the National Union of Teachers, said he strongly suspected that there was a single unofficial list of banned subjects. "The list is built on the assumption that these subjects are easier than others and not academic enough," he said. "This is just another sign of the Russell Group using a filter to stop people they don't want from getting into their universities. They have no concern about fairness. They should be far more transparent. If they have this list, let them publish it and show us the evidence that these subjects are easier."

Andy Gardner, of the Institute of Career Guidance, said he had confronted Russell Group universities about the alleged list in the past. "I think there is certainly an element of there being subjects that [the Russell Group] doesn't rate," Gardner, who advises state school pupils on their university choices, said. "Children in state schools are disadvantaged by this."

Mike Griffiths, headteacher of Northampton School for Boys and a council member of the Association of School and College Leaders, said he suspected universities of rejecting students who did drama A-level in particular. One of his pupils has three A*s but no place at university and he believes this may be because he took drama. "Universities need to be more honest about what criteria they are using [to select pupils]," he said. "I don't have a problem with universities having subjects that they consider to be less helpful, but they need to be upfront about it."

Geoff Lucas, secretary of the Headmasters' and Headmistresses' Conference, which represents headteachers of 250 independent schools, said universities must "come clean if there were unwritten rules" about so-called banned subjects. State school pupils were more likely to choose a subject in the unofficial list because independent schools mainly offer traditional subjects, Lucas said. "Students have the absolute right to see this list."

Wendy Piatt, director general of the Russell Group, said no Russell Group university barred any A-level subject.

"University websites typically include details on 'essential' and 'preferred' A-levels to help students maximise their chances of gaining entry to competitive degree courses," she said. "Most provide very clear and comprehensive information on required A-level subjects and which ones will not be considered when making admissions decisions. Students would be well advised to take very careful note of such requirements."

This week's A-level results showed that pupils were increasingly shunning so-called "soft" subjects in favour of science, economics and maths.