Top universities today denied having an unofficial list of "banned" A-levels, including less traditional subjects such as drama and business studies.
The Russell Group of 20 leading universities dismissed accusations that candidates who take subjects which appear on the list are rejected outright during the application process.
The existence of such a list could put state school students at a disadvantage because private schools are less likely to offer non-traditional subjects, which also include law and art and design.
A spokesman for Oxford University said: "There is no unofficial banned list of A-level subjects at Oxford. The only A-level subject we don't consider towards our standard offer of three As is General Studies.
"All the requirements are fairly and openly available on our website and we make huge efforts to go out and let people know about our application process."
A spokeswoman for Cambridge University said there were some "less preferred subjects" but to say they were banned was "absolutely not true".
She said: "As long as students are taking at least two traditionally academic subjects alongside any of the less-preferred subjects, their application will not be disadvantaged."
The university's admissions website says: "Before applying, you need to check that you have, or are likely to achieve, the right grades at the right level and in the right subjects for the course you have chosen."
A spokeswoman for University College London also denied the existence of a banned list, saying: "We are clear about what we do and don't accept and there is nothing hidden beneath that."
Wendy Piatt, director general of the Russell Group, said: "No Russell Group institution bars any particular 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.
"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 for their desired degree courses before picking supplementary A-levels."
The London School of Economics says on its website that it is always the case that "at least two traditional academic subjects are preferred".
An Economics candidate is advised: "Subjects where the content is deemed to overlap, such as Economics and Business Studies, or English and Media Studies, should not be taken together, and as with all degrees at LSE, traditional subjects are preferred."
John Bangs, former head of education at the National Union of Teachers, was among those who said he strongly suspected the existence of an unofficial list.
He told the Guardian: "The list is built on the assumption that these subjects are easier than others and not academic enough.
"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."