A poll of cancer specialists for the charity Breast Cancer Now found confusion over who funds bisphosphonates - cheap drugs typically used to treat osteoporosis.
The drugs alter bone tissue, making it harder for cancer cells to survive there, and are already used for patients with advanced prostate cancer.
They have been hailed as having the potential to prevent around one in 10 breast cancer deaths, and could save the NHS millions of pounds every year.
Breast Cancer Now calculates that around 27,000 women every year are missing out on bisphosphonates, with the main reason cited being a lack of clear guidance on who should fund them.
Its poll, which involved responses from 125 cancer specialists, found 59 per cent had not brought in bisphosphonates for their breast cancer patients. A quarter had introduced them and 17 per cent had done so partially.
When asked about possible barriers to giving the drugs, 45 per cent said they were waiting for funding decisions, 11% were waiting for clinical decisions and the rest were waiting for a variety of other reasons.
Baroness Delyth Morgan, chief executive at Breast Cancer Now, called for a national policy on who would fund the drugs, such as centrally through NHS England or via local clinical commissioning groups (CCGs).
She said: “While bisphosphonates are not routinely available to all eligible breast cancer patients, women’s lives are needlessly being put at risk.
“These are cheap and widely-available drugs and the overwhelming evidence of their ability to save lives should have changed practice by now.
“But they are still sitting on the shelf, blocked by bureaucratic inertia.
Research published in The Lancet medical journal last year found the drugs could prevent 1,000 breast cancer deaths a year.
Postmenopausal women on bisphosphonates saw a 28 per cent reduction in the chances of their cancer coming back. The drugs cut the risk of dying from the disease during the first 10 years after diagnosis by 18 per cent.
Analysis by Breast Cancer Now shows the cost of giving women the drugs would be almost £17 million a year but it forecasts eventual savings through women needing fewer bone scans and the fact that 1,200 fewer women every year would develop secondary breast cancer.
Bisphosphonates can be taken for around three years, either as a daily tablet (ibandronate) costing 34p per day, or intravenously every six months (zoledronate) at 4p per day.
Rob Coleman, professor of medical oncology at the University of Sheffield, said: “It should be an absolute priority for every clinician and commissioner involved in the provision of breast cancer services to ensure this simple and safe treatment is made available, as a matter of urgency.”
An NHS England spokesman said: “The report of the independent Cancer Taskforce included a specific recommendation on the use of bisphosphonates.
“We continue to work to realise the ambitions of the report and will shortly be publishing an update on progress made.”
NHS England said decisions on the funding of the drugs were currently made locally. But it said their use would be considered by the National Institute for Health and Care Excellence.