Making more cancer drugs available could cost far more than government estimates according to a BBC investigation.
The government has allowed for a £200m cancer drug fund to pay for more cancer treatments from next year.
But the cost could rise to £600m based on figures from drug manufacturers and the National Institute for Clinical Excellence (NICE).
The higher figure covers drugs turned down by NICE in the past five years.
Prof Chris McCabe, of the University of Leeds, said the cancer fund, which is due to start in April 2011, would not meet the need for access to cancer drugs.
"If you think about the cost of some drugs being £20,000 to £40,000 per patient for a year, you can get through £200m very quickly," he told Radio 4's The Report.
"It is going to be worse than it was before, because patients will believe the government has said they can have any cancer drug the doctor thinks they need and very quickly you will have patients being told by their doctor, 'Well I think you need it but that fund has run out," he added.
The BBC's figures were calculated using the prospective cost per patient of cancer drugs rejected by NICE over the past five years.
The number of patients was based on clinical criteria for prescribing the drugs from both NICE and the drug manufacturers.
More money for cancer, means less money for motor neurone disease. More money for cancer means less to treat patients who have got Alzheimer's
Health economist Professor Alan Maynard said he was concerned about the push for more cancer drugs.
"You are undermining NICE and all the work it does to target resources where we get the biggest health gain, the bang for the buck. And this drives a horse and cart through logical and systematic rationing of health care resources," he said.
Prof Maynard said he was worried that the cancer fund will see cancer being given preference over other conditions because a combination of drugs companies, patients and campaigners has put the government under pressure.
"More money for cancer, means less money for motor neurone disease. More money for cancer means less to treat patients who have got Alzheimer's."
He said many of the cancer drugs were portrayed as wonder drugs when they only extend a patient's life by three to four months.
"The pharmaceutical companies' PR has been first class."
Haematologist Professor Adrian Newland, from the Royal London Hospital, said he was also concerned that the pressure for cancer drugs will affect the relationship between doctors and patients.
"It will distort the doctor-patient relationship. Instead of being honest with a patient and about their prognosis, it will be easier to say: 'I've got this drug. Let's try it, when there could have been other cheaper treatments which would have a similar effect," he said.
No government minister or official would be interviewed by the BBC but in a statement the Department of Health later challenged the BBC's findings:
"It is unclear how the £600million figure was calculated. However we know that it fails to take into consideration the number of people who currently receive the drugs which NICE rejected.
It also fails to take account of individuals who may refuse a drug because they are concerned about the potential side effects.
The cancer drugs fund will be subject to an impact assessment soon. We support the £200 million figure. It factors in the number of people who actually requested the drugs but were not given them."