Universities recruit many modern language students from private schools
University courses important to the economy rely on independent schools for many of their students, says research.
These "strategically important and vulnerable" degree subjects include modern languages and engineering.
The study for an independent schools' group found a quarter of places in such subjects in leading UK universities went to independent school pupils.
Without such pupils the subjects would be at risk, argues the report by an Exeter University academic.
The analysis of university admissions, commissioned by the Headmasters' and Headmistresses' Conference (HMC), shows that independent school pupils are disproportionately represented in these economically important subjects.
Based on figures from 2006-07, the research says that 42% of undergraduate students entering economics in leading universities were drawn from independent schools.
Among modern languages, 28% of French degree students were from independent schools, with the figures 38% for Italian and 41% for Spanish.
In mechanical engineering the independent school entry was 26%, in civil engineering it was 25% and in general engineering 36%.
Comparing this with university entrance in 2003-04, report author William Richardson of Exeter University found the proportion of independent pupils in leading universities had either been maintained or had slightly increased.
Showing the wider context of these admission figures, about 9% of 17-year-old pupils are in independent schools - and 14% of university entrants are from independent schools.
Without independent pupils, "the study of subjects recognised to be vital to the future of the nation would be in serious jeopardy in many of our leading universities", says Headmasters' and Headmistresses' Conference chairman Andrew Grant.
Independent schools argue that the research shows the scale of their contribution to maintaining important degree subjects, identified by the government as valuable to the wider economy.
It also reflects the extent to which independent schools in England have continued to teach subjects such as modern languages at GCSE and A-level, when they are no longer compulsory in the state sector beyond the age of 14.
University modern languages departments have reported problems in finding sufficient applicants, as their potential pool of A-level students has diminished.
In economics, a report last year warned that the subject was at risk of "dying out" in schools - with A-level student numbers down by a quarter in a decade.
The research also follows a report earlier this year into social mobility by MP Alan Milburn, which interpreted the over-representation of independent school pupils in prestigious university courses as evidence of the weaknesses in the ambitions of the state sector and in university admissions.
Mr Milburn's report argued that even though participation in university had widened, children from wealthier backgrounds continued to dominate the most sought-after subjects at the most prestigious universities.
The report from the HMC shows that 38% of students entering medicine in leading universities are from independent schools.
A survey from the UCU lecturers' union, also released on Tuesday, claims that a majority of people want to see the ending of charitable status for independent schools.
It found that 56% of people wanted to abolish charitable status, including 41% of Conservative voters.
At the HMC's annual conference, Mr Grant dismissed claims that charitable status meant that independent schools were a cost to the taxpayer - arguing that private school fees saved the state sector £3bn, in terms of the private pupils who would otherwise have to be taught in state schools.
A spokeswoman for the Department for Business, Innovation and Skills said: "The government believes that it should be character, endeavour and talent that matter, not life chances at birth, which is why our policies have seen an increase in the proportion of people from less privileged backgrounds going into university.
"But we recognise that there is still more to do which is why we continue to invest in initiatives, such as Aimhigher, that encourage students from state schools to aspire to the most competitive courses and universities, and our recent white paper also set out our plans to enhance social mobility in the years to come."