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% had not brought in bisphosphonates for their breast cancer patients. A quarter (24%) had introduced them and 17% had done so partially.
When asked about possible barriers to giving the drugs, 45% 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.
“We’re fortunate to have so many of the world’s leading oncologists working within the NHS, and we must ensure they have all the tools they need to offer the best possible breast cancer treatment.”
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% 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%.
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.
The full cost (including consultant time and monitoring side-effects) is estimated to be 43p per day per patient.
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.”
Professor Ian Smith, chairman of the Breast Cancer Clinical Reference Group, said: “It is deeply concerning that such an inexpensive treatment known to improve survival for women with breast cancer is not being made available to all that could benefit.
“In order to deliver the best quality breast cancer care to UK patients, the introduction of these drugs for all eligible women must now be our top priority.”
The call comes as a separate survey for Breast Cancer Care found two-thirds (66%) of NHS hospital trusts in England did not know how many of their patients had secondary, incurable breast cancer.
Despite being mandatory since 2013, a fifth (19%) of hospital trusts are not collecting data about the number of people diagnosed with incurable breast cancer and almost half (47%) are not recording the numbers fully.