Medical students will face university debts of up to £64,000 if planned top-up fees get the go-ahead, the British Medical Association has warned.
Doctors usually study for six years at university
But Education Secretary Charles Clarke said the sums were "methodologically flawed" and "grossly excessive".
The BMA says undergraduates from all backgrounds would suffer, despite promises of grants for the poorest.
A London student on a six-year course, and whose parents earned £30,000, could run up a £64,661 debt, the BMA said.
The government wants to raise maximum annual tuition fees to £3,000 by 2006 from a current flat rate of £1,125.
MPs will vote on the proposals on Tuesday, in what is predicted to be one of the closest calls for Tony Blair's government.
Ministers have promised grants and bursaries for those from the poorest backgrounds.
But trainee doctor Fiona Pathiraja said: "It will have a massive impact.
"It will put off a lot of good candidates applying for medicine and make studying more of a struggle than it already is.
She added: "It's already a long course - we have to be a student for six years when lots of our friends have already graduated and are earning a good wage."
The BMA says a third of medical students in the UK take courses lasting six years and so face huge debts.
It calculates a student studying for a six-year course whose joint parental income was £15,000 could still leave university owing £51,642 if they studied in London, or £38,023 if they did so outside the city.
Daniel Gibbons, deputy chairman of the BMA's medical students committee, said: "Debts of this level would put anyone off becoming a doctor.
Charles Clarke dismissed the way the figures were calculated
"Medicine is already significantly more expensive than other courses, and our research shows that the problem would worsen with the introduction of top-up fees. "
But Mr Clarke dismissed the way the figures had been calculated.
"They indulge in double counting and ignore the increases in
grants and bursaries that will be available thanks to variable fees and will
help reduce overall debt for many low income students."
He said the government would pay the fees of medical students in years 5 and 6 of their courses and provide means-tested bursaries.
He added that the earning power of medical graduates would enable them to pay off their debts at a quicker rate.
"Doctors earn considerably in excess of the national average. Figures
collected in March 2003 show that 73% of newly qualified medical graduates in
hospital posts earn around £35,000 per annum.
The average starting salary for graduate-level jobs in 2002-03 was £18,000.
The BMA said its figures assume all medical schools are likely to charge the full fee rate.
It based its sums on students taking the standard six-year degree and calculated average expenditure from an e-mail poll of BMA medical students committee representatives.
It also used data published by the government and the Student Loans Company.
Liberal Democrat health spokesman Paul Burstow said the "short-sighted" policy would have serious long-term consequences for the NHS.
"Tuition fees will fuel the growing number of doctors who train here before either going to work in the private sector or going abroad to earn enough money
to repay these dreadful debts," he said.