Universities want the freedom to charge top-up fees and fear that the government's student-support review could rule them out.
Universities UK has warned the government that any proposed graduate tax would scupper the introduction of differential fees in the future and damage university finance.
In a submission to the government's review, UUK said tuition fees must stay. But it backed the reintroduction of maintenance grants, saying that student hardship was hampering efforts to widen participation. It proposes that wealthier families pay more to support their children through university.
Universities are aware that they could lose an important stream of funding - worth up to £400 million a year - if the government decides to replace fees with a graduate tax.
While differential fees are not UUK policy, support for the principle gives institutions options for the future and a frontline defence against a graduate tax. The UUK submission warns that the introduction of a graduate tax would delay for a generation a more market-driven system.
It refers ministers to the Taylor report on future university funding commissioned by UUK last year. That report says that allowing universities to charge "market fees" is one of several options for the sector.
Loss of tuition fees would leave institutions out of pocket and again dependent on centrally controlled teaching income.
Tony Bruce, policy director at UUK, said that a number of vice-chancellors at UUK were in favour of a "rapid move" to charging top-up and differential fees. He said, however, that the organisation's commitment to maintaining up-front tuition fees was determined more by a concern to protect universities' immediate income than by an enthusiasm to pave the way for top-up fees in the future. "Our members would be concerned that this income could disappear under a transition to a graduate tax or other deferred-payment system. We would be left at the mercy of public funding for a long transitional period.
"A graduate tax would make a market-based fee system more difficult because the charge would be applied after graduation and you could not apply differential charges at that point."
In addition to keeping tuition fees, the UUK paper says there are a number of other firm principles that should underpin any new student-support system. The paper, by consultant Nigel Brown Associates, says there should be "substantially increased support for maintenance in place of loans for students from the poorest families".
It says that the system should be simplified to help students understand the support available to them, and there should be an "increased contribution (towards maintenance) from those from the most well-off families".
UUK's support for increased student maintenance grants does, however, represent something of a relaxation of the vice-chancellors' view that any increase in support for students will come at the expense of cash for university teaching and research. It reflects a growing concern among vice-chancellors that the expansion required to hit the government's 50 per cent participation target by 2010 may be undermined unless more is done to remove perceived financial deterrents to the poor.
UUK's submission argues that although up-front tuition fees have been scrapped in Wales and Scotland, they have been wrongly highlighted as the main problem. The report says they are "the most progressive element of the current package of student finance". It says there is a "widespread misunderstanding" among poor students who are unaware that they are exempt from tuition charges.
The Department for Education and Skills, which is leading the review, is preparing to hand its student-support blueprint to Downing Street and the Treasury for consideration.
A consultation document is due to be issued in spring and a final report with recommendations is expected later in the year.