Foreign students are propping up British universities in key subject areas like science and engineering, a report for UK vice-chancellors says.
More than a fifth of students in subjects deemed "strategic" by the government come from overseas, the Universities UK (UUK) report found.
Only 29% of postgraduate students in these areas are from the UK, with more than half coming from non-EU countries.
The government says attracting foreign students was a matter of satisfaction.
But Universities UK says more home grown researchers are needed.
Strategic subjects are defined as those which are vital on the grounds of wealth creation, diplomacy, international relations and cultural grounds.
They include science subjects, mathematics, technology, engineering and languages.
Chairman of UUK's long term strategy group Professor Geoffrey Crossick said the lack of UK-born postgraduate students was leading universities to fill their posts with foreign applicants.
The figures underscored "the importance of international students, not only for the financial health of UK higher education, but also for the renewal of disciplines in many areas and in underpinning the UK's world-class research base."
He added: "The concern is that should people return home, the flow of researchers will dry up and that will cause problems for the UK economy.
"The important thing for UK universities now is not to have fewer researchers from abroad but to build up more from the UK."
There are also fears that the lack of home grown researchers could add to the shortage of specialist teachers in these areas in schools.
There are particular concerns about the numbers of students studying physical sciences, within which chemistry has fallen by 19% and physics by 6% over the past 10 years.
The drop-off has led to the closure of a number of university science departments in recent years.
Prof Crossick said: "A key part of the problem with strategic subjects relates directly to the lack of student demand, and more must be done in schools and the wider society to address this."
At the same time the number of students studying for degrees in "trendy" subjects like psychology, (up 113%) computer science, law, media studies, drama and film (up 335%) has increased.
But perhaps surprisingly, the report added that university graduates with degrees in many of these strategic areas were more likely to be out of work than other graduates.
While the unemployment rate across all graduates in 2005 was 6.6%, for those with degrees in the strategic subjects, it was 8.4%.
This goes against claims that businesses are finding it increasingly difficult to find staff with the appropriate science and engineering qualifications.
A recent survey by the Confederation of British Industry suggested 80% of engineering and industrial companies and 67% of energy, water or utility companies were expecting a shortfall in overall graduate recruits this year.
The CBI suggested a "golden carrot" bursary of £1,000 a year should be given to all science and engineering graduates to help pay tuition fees and tackle the problem.
Higher education minister Bill Rammell said: "Higher education is a truly global activity. Success for our universities in recruiting international students and attracting international academics is a matter of satisfaction, not of concern.
"But it is certainly important to build up levels of scientific education throughout the UK population and workforce. Over the next three years we are spending an additional £75m to support provision of key science subjects in English universities.
"It should also be noted that the UCAS application figures for this academic year show increases for accepted applications in a number of strategically important subjects.
"The government is committed to encouraging more people to take up research careers, and this includes our focus on making the career choice a more attractive one and the career more secure."