Students are being warned they could narrow their chances of studying at top universities by taking non-traditional A-levels.
Admissions tutors find it hard to distinguish between the most able
The Russell Group of top universities said pupils at some state schools put themselves at a disadvantage by studying so-called "softer subjects".
With more A grades awarded at A-level, universities say it is hard to distinguish between top candidates.
No subjects are thought to be a "bar" to a course, but some are "less ideal".
Russell Group director general Dr Wendy Piatt said institutions were increasing the information provided for applicants about the qualifications needed to get on to particular courses.
"They now offer clear recommendations on the package of A-levels (or equivalent) which would give the candidate the best grounding for a particular course and which would be a less ideal combination of A-levels."
Most were posting "entry profiles" on the website of admissions service, Ucas, giving detailed guidance on how they might best tailor their post-16 study to their higher education needs, she said.
Competition was "fierce" for places at top universities - with many candidates for courses such as medicine, English and law having three or four A grades at A-level.
This made it difficult for admissions tutors to choose between such excellent candidates, she said.
CAMBRIDGE UNIVERSITY'S 'LESS IDEAL' A-LEVELS
Art and design
Design and technology
Drama and theatre studies
Health and social care
Information and Communication Technology
Travel and tourism
"Clearly if pupils from certain state schools are increasingly taking a combination of subjects which put them at a disadvantage in competing for a course at a Russell Group university, the task of widening participation in our universities becomes even more difficult," she added.
A spokesman said it was all about ensuring candidates studied the right A-levels for the particular course they want to study.
But some universities go further than this.
Cambridge University, for example, advises pupils not take more than one of a list of 20 A-level subjects, including art and design, dance, film studies and media studies, as part of the three A-levels normally needed to obtain a place.
Applying with two of these A-levels "would not normally be considered to be acceptable", it says.
A Ucas spokesman said: "The key thing about admissions is for universities to be transparent and we certainly encourage them to do that."
Schools Minister Jim Knight said: "We simply do not recognise the label 'soft' or 'hard' A-levels - all subjects are rigorously measured against each other to maintain standards."