A comprehensive analysis of a drug known as tamoxifen found that it could reduce the risk of cancer by 38 per cent in women who were genetically predisposed to the disease.
While the drug is currently used to treat some cases of breast cancer, it is not available to women who have not yet been diagnosed because of concerns about treating patients unnecessarily.
Now the new study has added to calls for the drug to be prescribed to women with a high genetic risk of the condition, after finding that it continues to benefit patients long after treatment has stopped.
Tamoxifen is currently used to treat the "oestrogen receptor positive" form of breast cancer, which accounts for almost four in five of the 50,000 new cases diagnosed in Britain each year.
But like many cancer drugs the treatment is toxic and can cause severe side-effects including blood clots, leading to concerns that the harms of administering it before diagnosis could outweigh the benefits.
Now researchers have found the drug could be twice as effective as previously thought, and its effects last for longer than formerly believed. The data suggested tamoxifen could prevent one case of cancer for every 42 high-risk patients treated.
Earlier this year the national drug regulating authority, Nice, issued draft guidance saying tamoxifen should be approved for prevention as well as treatment in women at high risk of the disease, but a final decision will not be made until later this year.
If approved, the drug could be offered to up to 500,000 women, some of whom may otherwise have chosen to undergo a preventive mastectomy, or breast removal.
The new study, published in the medical journal The Lancet, found that tamoxifen and three similar "selective oestrogen receptor modulator" drugs could reduce incidence of breast cancer in high-risk patients by 38 per cent.
The researchers, from Queen Mary, University of London, studied the health records of more than 83,000 women to examine the effectiveness of tamoxifen and the three other drugs, which are used to treat osteoporosis.
During five years of treatment the incidence of breast cancer among women using the drugs was 42 per cent lower than in a similarly high-risk group who were not treated.
Women in the treated group were also 25 per cent less likely to develop breast cancer in the five years after finishing their course of drugs, although there was no effect on the number of women who died from the condition, researchers found.
Prof Jack Cuzick, lead researcher, said: “These are very encouraging results and pave the way for more widespread use of these drugs in high risk women in a manner similar to the way statins and blood pressure lowering drugs are used to reduce the risk of heart disease and stroke.”
Hazel Nunn, of Cancer Research UK, said: “These results provide some of the clearest evidence to date of the ability of these drugs to prevent breast cancer.
“We look forward to the final guidance from NICE and hope this mark the time when women have more options to reduce their risk of breast cancer. These drugs have a range of sideeffects though, so they will not be suitable for all women."