Doctors in Oxford fear a new system of GP funding could reduce their income by 40 to 50 per cent.

About 10 practices in the city centre are worried they will lose out because of the high number of students on their books.

Under the current system, GPs are paid a fixed sum for each registered patient, but the new GP contract's financial formula takes factors like age, gender and health of patients into account.

While funding for elderly patients is high because they rely heavily on health care, payment for students is lower, as they are generally in good health.

Dr Gordon Gancz, a partner at the St Edward's Street practice, said students made up nearly half his list, making him much worse off under the new contract.

He said: "We provide a fairly unique service, because we look after lots of undergraduates, but earnings under the new contract would make our practice non-viable.

"On average, students contact their GP three times more than other patients, because they're away from home.

"There are a lot of issues they have to deal with, like contraception, sexually transmitted disease and the stresses of study.

"Far from being the people who never go to the doctor, they actually end up going to the doctor an awful lot, and the contract formula is wrong.

"I'm worried this could force any of us out of business. I know they're trying to rectify the problem, but I'm very irritated."

The NHS Confederation, which represents health service managers, published the GP contract in March after negotiations with the British Medical Association.

At first, the document was praised for giving GPs more money according to the work they carried out. But on closer inspection, it became clear that many would suffer cuts in funding.

Dr Rickman Godlee, of the Church Street Surgery, in Wantage, who is chairman of the Local Medical Committee and a member of the BMA's General Practitioners' Committee, said: "In terms of the way the formula works, all practices in Oxfordshire would have been losers.

"A lot of practices, especially those in the city centre, are very concerned about how this new contract will look for them and certainly when the formula was first announced they were going to be very badly affected."

He said the NHS Confederation and BMA had now negotiated a revised formula, called the Minimum Practice Income Guarantee (MPIG), to make sure no GP surgeries would lose out. A national conference is being held on Wednesday next week, to discuss the proposed changes and to decide whether they should be put to a ballot of GPs.

Dr Godlee said: "A new spreadsheet has been devised so GPs can calculate what they will get with the MPIG. They will then be in a position to assess whether the contract will work for them or not."