Foreign students are being charged up to four times more for degree courses than those from the UK, according to new research.
In some cases, foreign undergraduates are being asked to pay up to £35,000 for their courses, figures compiled by the Complete University Guide show.
The NUS warned that overseas students should not be treated as 'cash cows'.
Tuition fees for UK and EU students were trebled last year, and universities can now charge them up to £9,000 per year for an undergraduate degree.
But the Guide's new Reddin Survey of Tuition Fees suggests a broader range of fees for students outside the EU.
Undergraduate degree fees for 2013/14 vary between £7,450 and £35,000 for overseas students, the survey claims, depending on the type of course - classroom based, laboratory or clinical - that the student is taking.
Medical-based, or clinical are among the most expensive, it suggests.
A total of nine universities will be charging foreign students over £30,000 for an undergraduate clinical degree, the survey claims, with King's College London setting its tariff at £35,000.
One further institution - Edinburgh - is asking overseas students in the third, fourth or fifth year of a medical degree for £36,600, it suggests.
Recent figures for UK and EU students show that in 2014/15, the average tuition fee is expected to be around £8,600.
More than a quarter (27 per cent) of institutions will charge the maximum £9,000 as standard in 2014/15 and almost three in four (72 per cent) will charge the top price for at least one degree course.
Scottish universities ask for a standard £1,820 fee for all Scottish and EU students, the survey said.
The new survey also gives details of postgraduate fee levels - which are not subjected to the same fee level controls.
It suggests that taught postgraduate courses range in price from £2,000 to £27,552 for home and EU students, and between £7,900 to £38,500 for overseas students.
MBA (Master of Business Administration) courses can cost more, the survey claims. The most expensive is Oxford's one-year MBA at £41,000. This applies to both home, EU and overseas students.
Dr Bernard Kingston, founder of the Complete University Guide said: "The market in postgraduate and international student continues, and predictions of a similar trend at undergraduate level have been proved to be true. While there is a plethora of scholarships and bursaries intended to offset the impact of higher tuition fees on access by under-represented groups, the fact is that most institutions impose the full fee on home and EU undergraduates.
"Many students will not be eligible for scholarships and bursaries and will end up paying that full fee, probably through the loan mechanism which defers repayment until after graduation when the repayment threshold is met."
Daniel Stevens, NUS international students' officer, told the Times Higher Education magazine that the difference in cost for home and overseas students was "scandalous".
"International students are an important part of the social, cultural and academic make-up of university life and should not be treated simply as cash cows."