Three quarters of cancer doctors are allowed to prescribe Herceptin for early stages of the disease despite it not being licensed, a BBC survey says.
The drug is already used for advanced cancer
The Panorama poll of 390 oncologists in England and Wales found 28% were always allowed to prescribe the drug by their NHS trusts and 50% were sometimes.
Herceptin is only licensed for advanced stages, but pressure has mounted on NHS bosses after high-profile challenges.
The first court case against a decision not to fund the drug starts on Monday.
Ann Marie Rogers, a 53-year-old mother of two, is taking Swindon Primary Care Trust to court over its refusal to fund the drug.
It targets the HER2 protein, which can fuel the growth of breast tumours
Herceptin prevents this process happening
Around a fifth of breast cancers are HER2 positive
It is currently licensed for use in women with advanced breast cancer - where the disease has spread within the breast or to another organ
Early stage breast cancer refers to the first occurrence of the disease
The cost for one year's treatment with Herceptin is £19,500
She told Panorama: "I think it's down to money and I think they put money before life."
It follows previous legal bids last year that resulted in NHS trusts backing down before the cases reached court.
Nurse Barbara Clarke, 49, threatened Somerset Coast PCT with a judicial review, but the trust backed-down before the case and provided her with the treatment.
In November, Health Secretary Patricia Hewitt intervened when North Stoke PCT refused to fund the drug for mother-of-four Elaine Barber.
Ms Hewitt said she wanted to see the evidence up on which health bosses had made their decision and within a day the trust had reversed the decision.
At the time, the NHS Confederation, which represents health service managers, said NHS trusts were in an "impossible position" and would find it hard to refuse to fund the drug.
Panorama said the survey of cancer doctors showed that "patient power" was having an affect on PCTs, which are in charge of funding local health services.
Ms Hewitt said the decision on whether to prescribe the drug was still a matter for local decision making.
"It is already possible for a clinician to prescribe a drug for an unlicensed indication.
"The clinician will make this decision after discussion with the patient about the potential risks and taking into account medical history.
"However, if the drug is unlicensed, PCTs may also become involved to decide whether to support the clinician's decision and supply the unlicensed drug at NHS expense."
The programme, to be shown on Sunday, also deals with the North Stoke case, talking to health officials about the pressure put on them by Ms Hewitt's intervention.