Paying for the cancer drug Herceptin means treatment for other cancer patients will have to be dropped to "balance the books", doctors warn.
Herceptin was approved for NHS use in August
The Norwich team says the body which recommends drugs for the NHS should change the way it works.
In the British Medical Journal, they say the National Institute for Health and Clinical Excellence should say what should be cut to fund new drugs.
But the government said it was up to local trusts to plan effectively.
In August, NICE ruled that primary care trusts should pay for patients to have Herceptin (trastuzumab).
It followed a sustained campaign by breast cancer charities and patient groups to win access to the drug.
Herceptin targets the HER2 protein which can fuel tumours. Around 20% of breast cancers are HER2 positive.
NICE has said breast cancer patients with HER2 positive cancers who had a moderate or high chance of their cancer returning should be given Herceptin after they have had surgery, radiotherapy or chemotherapy.
'Treatment of the moment'
But the doctors at the Norfolk and Norwich University Hospital NHS Trust and the University of East Anglia said NICE provided no extra funding when it made its ruling and does not suggest what cuts should be made to release the extra money that is needed.
The team looked at the number of cancer patients they treat at the hospital, and calculated that they will have to find £1.9m each year to make Herceptin available to eligible patients.
Testing and monitoring costs would add to the bill, they said.
They audited drug costs in their hospital's cancer centre, and estimated how they could save £1.9m by cutting chemotherapy and palliative care treatments.
That sum would pay for 75 patients to receive Herceptin.
Funding the drug would mean 355 patients not receiving post-surgery treatment - 16 of whom would be cured, or around 200 patients not receiving palliative chemotherapy.
The researchers say there is little long-term data on the clinical benefits and potential dangers of taking Herceptin, whereas the established treatments have been shown to be clinically and cost effective.
'A universal problem'
In the BMJ, the researchers said: "These untreated patients will be people we know.
"We will be the ones to tell them they are not getting treatment that has been proved to be effective and which costs relatively little, because it is not the treatment of the moment.
"These results are obviously not definitive, but illustrate the fundamental challenge facing the NHS - the tension between national priority setting and local implementation."
Dr Tom Roques, a cancer specialist who worked on the study, said: "In an ideal world, there would be ring-fenced government money to back up NICE decisions.
"But we at least need a public debate about how we keep up with new treatments and expensive treatments in a rationed NHS.
Professor John Toy, medical director of Cancer Research UK, said: "It's not fair for doctors and hospitals to have to decide what treatment or services should be cut to make ends meet. And most importantly it's totally unacceptable for any patient who might not receive the best treatment available."
Dr David Jenner, of the NHS Alliance which represents primary care trusts, said: "This is a problem that every PCT faces.
"If NICE says something has to be funded, it does put pressure on the budget and something has to give way."
He added: "The Department of Health should keep a central pot of money to fund NICE decisions."
But health minister Rosie Winterton said: "Doctors treat patients according to their clinical need. It is quite wrong to say that Herceptin is the only drug that is likely to be funded.
"PCTs should always be planning ahead and we would expect them to consider the implications of introducing all drugs on the horizon, not just Herceptin."