The National Union of Students says England's system for funding higher education is unfair and reinforces existing social inequality.
In a report timed to influence the terms of a government review of the system, it says things will get worse if the cap on top-up fees is raised.
Financial support is based on where you study, not on need, it says.
Variable or "top-up" fees began in 2006 and are capped at £3,145 a year - but could rise in the future.
Parliament could vote for an increase after 2010. The Westminster government has said it will hold a review of the funding system once the first people to be affected by the changes have passed through the system.
It expects to finalise the terms of the review late next year.
In its report, Broke and Broken, the NUS says students will be faced with "staggering" debts if the current cap is raised substantially.
It is critical of the current system of bursaries offered by individual universities (which is on top of government support) and calls for a national system instead.
NUS president Wes Streeting said: "Market forces have already crept into our higher education system.
"More prestigious universities in the Russell Group are able to offer poorer students an average annual bursary of £1,791, but less prestigious universities are only able to offer £680.
"There is clearly a market of prestige at work, with financial support being based not on how much you need it but on where you study."
'Old arguments'
The union has decided to stop campaigning for fees to be abolished.
Mr Streeting said the report "was not for special pleading on the principle that education should be free".
"I believe it should be - but I know the debate has moved on and we won't win by dredging up the old arguments," he said.
The report says that if top-up fees were raised to £7,000, some students would end up owing almost £40,000 on graduation.
Higher Education Minister Bill Rammell said: "The new system is working, as is demonstrated by record levels of applications and acceptances, up by 6% this year.
"The present financial system already provides a guaranteed level of support for all students "
"Record numbers of students from all social classes are choosing to go into higher education and reap the benefits this brings.
"This is at a time when the government has abolished upfront fees and this year two thirds of students will benefit from a full or partial grant of up to £2,835.
"We have always stated that we will meet our commitment to have an independent review of the first three years of variable fees. I would not like to pre-empt the findings of this review as this report has done."
Director general of the Russell Group of leading universities, Dr Wendy Piatt, said there was undoubtedly a need for a debate on university funding.
To stay internationally competitive, universities would need to increase funding from public and private sources.
'Misguided'
Dr Piatt said there was no evidence that a national bursary system would widen participation.
"In fact it is more likely to hamper all the efforts Russell Group universities are making to encourage students from non-traditional backgrounds to apply," she said.
"There is evidence that targeted, generous bursaries have helped to attract very able students from low-income backgrounds to universities they may not otherwise have considered."
"One could understand the call for a national bursary scheme if there were no assistance available for students in need. But this is just not the case.
"The present financial system already provides a guaranteed level of support for all students with much higher support offered to disadvantaged students through government grants, loans and no upfront fees."
To suggest it was somehow wrong for universities to top up this support with bursaries that far exceeded the amount originally required by the Office for Fair Access seemed "profoundly misguided", she said.
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