Pupils are being advised to avoid taking certain A-level subjects if they want to get places at top universities.
Cambridge has issued advice on which A-levels to avoid
Cambridge University and the London School of Economics (LSE) say traditional subjects offer the best preparation for study at degree level.
Subjects less favoured by the two institutions include accounting, business studies, film studies, media studies and travel and tourism.
Cambridge said it was giving "explicit" advice on subjects for the first time.
Informally, it has been left to teachers or savvy parents to advise post-GCSE candidates on what subjects carried most weight at A-level.
But Cambridge is now issuing a list of less suitable A-levels in its online prospectus, saying too many applicants are not getting appropriate advice.
The university lists 20 subjects "that provide a less effective preparation for our courses".
"To be a realistic applicant, a student will normally need to be offering two traditional academic subjects (i.e. two subjects not on this list).
"For example, mathematics, history and business studies would be an acceptable combination of subjects for a number of our courses.
"However, history, business studies and media studies would not normally be considered to be acceptable as this combination contains only one subject not from the list below."
A-levels in general studies and critical thinking "will only be considered as fourth A-level subjects and will not therefore be accepted as part of a conditional offer".
The university also lists five subjects, such as business and management and theatre arts, which are considered less effective at the higher level of the International Baccalaureate.
The LSE takes a similar line, saying it prefers certain subjects because they are "more likely to serve as effective preparation" for study there.
Its online prospectus sets out a dozen - including accounting, media studies and law - only one of which will count for course entry purposes.
LSE says traditional subjects offer the best preparation for study there
"For example, mathematics, French and economics would be a suitable combination for almost any of our degrees.
"Mathematics, French and business would be acceptable, but we would prefer the first example.
"On the other hand, mathematics, accounting and media studies would not normally be considered as suitable as this combination includes two subjects on the list."
Like Cambridge, general studies and critical thinking will not count towards the requirements of any conditional offer.
"Similarly, an A-level or equivalent in your first language may not be counted," the prospectus says.
A spokesman for Cambridge University said: "It is the first time we have been so explicit.
"We have previously been explicit in identifying which A-level subjects were desirable or less desirable for given courses, but less explicit in giving guidance as to which A-levels provided good or less good preparation for our courses more generally.
"We have moved to do this because it was becoming increasingly apparent that some students were not getting good advice from schools, careers advisers, etc."
A spokesman for the Department for Education and Skills said: "There is no such thing as a 'soft option' at A-level."
"All A-levels have strict standards which have been set by the awarding bodies and monitored by the Qualifications and Curriculum Authority."
Universities had always set criteria on the combination and grade of subjects required for admissions for certain degree courses, he added.
"For example you wouldn't generally study A-levels in languages and then apply for a degree course in medicine.
"This is simply about all students needing to choose the right combination of subjects as preparation for particular degree courses."