The drug fulvestrant (Faslodex) has not been recommended in England to treat late stage breast cancer, in draft guidance.
The National Institute for Health and Care Excellence (NICE) has provisionally rejected fulvestrant for post-menopausal women with a form of the disease known as oestrogen-receptor positive.
Professor Arnie Purushotham, Cancer Research UK's senior clinical adviser, said that the decision was disappointing.
"Trials have shown that it can stall the cancer's growth, although the results don't yet show that it can improve survival," he said.
"We hope the manufacturer and NICE can work together to reach an agreement which allows the drug to be made available when NICE issues its final guidance."
The guidance covers women whose breast cancer is at a later stage or has spread to other parts of the body.
Fulvestrant works by blocking the hormone oestrogen, stopping its ability to cause some breast cancers to grow.
It would be used for postmenopausal women who haven't previously been treated with hormonal therapy such as aromatase inhibitors or tamoxifen.
Of the 32,000 postmenopausal women diagnosed with breast cancer each year in England, it's estimated that up to 1,200 would be eligible for treatment with fulvestrant.
While NICE acknowledged that it can stall tumour growth by up to three months compared to aromatase inhibitors, it said early evidence isn't strong enough to show that the drug extends survival.
A final decision will be made later in the year.
NICE said cheaper treatments are available which have more evidence for increasing survival.
The average cost of a course of fulvestrant is nearly £16,000.
Professor Carole Longson, director of the centre for health technology evaluation at NICE, said that while better treatments are needed for advanced breast cancer, NICE has to ensure that those treatments offer value for money.
"As fulvestrant has not been shown to be cost-effective, we can't justify diverting NHS funds from other areas of healthcare in order to fund its use," she added.
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Fulvestrant for untreated hormone-receptor positive metastatic breast cancer [ID951]. www.nice.org.uk/guidance/indev … ltation/html-content