Gordon Brown will tomorrow join a campaign by cabinet ministers to rally support for top-up fees - as doctors' leaders warned the legislation could leave medical students with debts as high as £64,000.

The Chancellor, who has been accused of showing reluctance to offer his support, will be one of several ministers touring television and radio studios to rally support for the Bill, after Alan Johnson, the Higher Education minister, conceded yesterday that the Government could lose the vote.

The bleak assessment came as the British Medical Association (BMA) published research showing that debts could rise by as much as 44 per cent if the legislation is agreed - prompting would-be medical students to decide against becoming doctors. BMA officials said they calculated the debts on the basis that all medical schools were likely to charge the maximum top-up fee of £3,000 a year for their six-year courses.

The highest debts were likely to be incurred by students in London who failed to qualify for new grants and bursaries. A medical student on a six-year course who qualified for grants could leave owing a maximum of £38,023 outside London, or £51,642 in the capital.

The figures were dismissed by Charles Clarke, the Secretary of State for Education, as "grossly inflated". But Daniel Gibbons, the deputy chairman of the BMA's medical students committee, said: "Debts of this level would put anyone off becoming a doctor. 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."

The warning came as Downing Street acknowledged that the Chief Whip, Hilary Armstrong, had warned the Cabinet could lose Tuesday's vote debate by 30 votes.

In a taste of the increasing intemperance in the run-up to the vote, Mr Johnson accused some rebels of trying to bring down Tony Blair. "I'm not going to put a name to any of those," he said. "Certainly I don't think any of them are interested in the issues they profess to be interested in." Rebel leaders insist they still have at least 100 firm declarations of support and believe 20 MPs will abstain - enough to inflict defeat on the Government. But ministers took heart yesterday with the first indication from a university that it would not increase its fees if the legislation came into force. Thames Valley University said it was not expecting to increase its fees, although a decision had yet to be taken.

* Oxford University is facing the threat of legal action from the Association of University Teachers over a decision to withdraw facilities allowing it to recruit new members during a pay dispute. The union is holding a ballot over strike action in protest at a pay offer of 6.44 per cent spread over two years.