University tuition fees in England should rise in line with inflation, a university leader has urged.
Sir Christopher Snowden, vice-chancellor of the University of Surrey and president of Universities UK, says a £9,000 fee cap is "not sustainable".
"The whole issue is that fees can't remain frozen for ever", he told Times Higher Education magazine.
The National Union of Students (NUS) said last year's trebling in fees had already far outpaced rising costs.
Sir Christopher's comments follow a recent call from the vice-chancellor of Oxford University, Andrew Hamilton, for undergraduate fees to be "more closely related to the true cost" of a degree at Oxford, £16,000.
Last month Prof Hamilton said the funding shortfall meant Oxford faced an annual funding gap of £70m.
Sir Christopher said that Surrey had "several subjects where we are losing substantial sums of money teaching UK and EU students".
He said these included science, technology and engineering courses.
Sir Christopher added: "On aggregate the £9,000 was perhaps a reasonable starting point, but it really needs to have a sensible indexing linked to it."
He said that the £9,000 maximum fees paid by students would be worth only around £8,250 by 2016 when inflation was factored in.
However, Sir Christopher told the magazine that he was not an advocate of a major jump in fees: "I don't think that £16,000 is a likely scenario in the near future."
Universities UK will publish a report next week on the future of university financing.
This is expected to look at how universities will fund a predicted 26% rise in demand for university places within the next 20 years.
Universities Minister David Willetts has said a rising birthrate will mean the number of university places needed will grow from 368,000 to 460,000.
The paper is expected to explore where universities will find the capital to expand their facilities and accommodation.
Universities UK points out that the rise in tuition fees paid by students was accompanied by a corresponding cut in the level of government funding for universities.
'Huge financial squeeze'
The NUS said suggestions that undergraduates should pay more were unlikely to win sympathy from students or their parents.
Union president Toni Pearce said: "What seems to have been missed here is the inflation-busting increases on the upper limit on tuition fees just last year, when the cap was nearly tripled to an eye-watering £9,000.
"We've already seen a huge shift onto the shoulders of individuals through higher fees that far outpace rising costs.
"Students and their families are facing a huge financial squeeze. The value of both earnings and student financial support is falling in real terms and the cost of making ends meet is increasing by the day."
The government said its reforms had put the higher education sector on a more sustainable footing.
A spokesman for the Department for Business, Innovation & Skills said: "We believe the £9,000 fee allows universities to deliver high-quality teaching, with many institutions now planning to invest more in improved teaching facilities for students.
"We want to ensure that going to university remains affordable for students and it is based on their ability, not their ability to pay."