Scientists have expressed their disappointment over the health watchdog's preliminary decision not to offer a life-extending drug to prostate cancer patients until after they have received chemotherapy.
Experts from The Institute of Cancer Research (ICR) said that abiraterone should be offered to men before they undergo chemo.
The drug could offer many men not eligible for chemotherapy additional months to live with improved quality of life, they said.
They accused the National Institute of Health and Care Excellence (Nice) of making its decision on the basis of costs.
In new draft guidance, Nice said that the drug, also known as Zytiga, should not be recommended to treat prostate cancer which has spread in people whose first treatment has failed, have no or mild symptoms, and for whom chemotherapy is not yet clinically indicated.
A spokeswoman said that Nice already recommends the medication, made by manufacturers Janssen, for patients who have already undergone a certain type of chemotherapy.
Professor Paul Workman, deputy chief executive of The Institute of Cancer Research, London, said: "We are very disappointed that Nice has not felt able to recommend use of abiraterone for men with prostate cancer who are yet to receive chemotherapy.
"Abiraterone, which was discovered and developed here at The Institute of Cancer Research, is now used as standard after chemotherapy and has extended the lives of thousands of men in the UK with advanced prostate cancer, with fewer side-effects than chemotherapy.
"The decision to refuse use of abiraterone before chemotherapy will deny many thousands of men the opportunity to access the drug earlier in their course of treatment. It is a particular blow for men who are unable to have chemotherapy because they are too old or too frail, since they could now miss out on abiraterone too.
"We believe the evidence is clear that abiraterone is effective when given before chemotherapy and can give men many months of extra life free of disease, and improved quality of life.
"Nice appears to have made its decision on the basis of cost, and we recognise that it does have a responsibility to achieve cost-effectiveness for the NHS. It is disappointing that abiraterone was considered too expensive to be recommended before chemotherapy, and we would urge Nice, the NHS and the manufacturer to reach an accommodation on the drug's price as soon as possible, so that it can be made more widely available."
Nice's chief executive Sir Andrew Dillon said: "Although abiraterone is not a new drug, it is the first treatment to become available at this point in the treatment pathway. We know how important it is for patients to have the option to delay chemotherapy and its associated side effects, so we are disappointed not to be able to recommend abiraterone for use in this way. However, the manufacturer's own economic model showed that the drug would not be cost-effective at this stage - because of this we cannot recommend the drug in this preliminary guidance.
"We hope that during this consultation, the manufacturer uses this opportunity to look again at its submission and provides the committee with additional information which may enable them to reconsider."
Mikis Euripides, director of policy and strategy at charity Prostate Cancer UK, said: "This news is a huge blow to those with advanced disease who have long hoped for the chance to delay chemotherapy and the debilitating side effects and incessant hospital visits which come with it.
"In the 16 months since it became available on the Cancer Drugs Fund in England, pre-chemotherapy abiraterone has become the fund's second most requested drug, indicating great confidence from clinicians about its benefits at this stage of the disease and great value from patients who, with input from their clinicians, are opting to delay or avoid having chemotherapy.
"It seems unforgiveable that such a powerful and beneficial treatment should be left tantalisingly out of reach.
"We implore Janssen and Nice to work together to do everything in their power to make this vital treatment available as soon as possible for all in England and Wales who need it. This decision has been delayed once before and it is more than frustrating that still we seem to be no further forward. Men should not be forced to wait a moment longer."
A spokeswoman for Janssen said: "Janssen is disappointed that the Nice appraisal consultation document published today does not recommend the use of Zytiga (abiraterone acetate) for the treatment of men with advanced hormone relapsed prostate cancer who have not previously been treated with chemotherapy.
"Janssen will now work constructively with Nice to address the concerns they have raised at this stage in the process."