Researchers say the female hormone progesterone could be included in the treatment of breast cancer because it slows the growth of tumours.
Half of breast cancer patients could benefit from having the widely available female hormone progesterone added to their treatment, research suggests.
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The hormone is involved in signals between cell molecules that can hold back tumour growth, scientists have found.
Experts have long been puzzled by the fact that breast cancer patients whose tumours have progesterone receptors - molecules sensitive to the hormone - often have a better outlook.
The new study shows how the progesterone receptor "talks to" another receptor sensitive to the hormone oestrogen, which fuels breast cancer in a large number of cases.
This has the effect of applying a brake on the oestrogen receptor's ability to stimulate tumours.
Dr Jason Carroll, from the Cancer Research UK Cambridge Institute, said: "We've used cutting-edge technology to tease out the crucial role that progesterone receptors play in breast cancer - a mystery that has baffled scientists for many years.
"This important laboratory research helps explain why some breast cancer patients have a better outlook.
"Crucially, it provides a strong case for a clinical trial to investigate the potential benefit of adding progesterone to drugs that target the oestrogen receptor, which could improve treatment for the majority of hormone-driven breast cancers."
The findings are reported in the journal Nature.
About 50,000 new cases of breast cancer are diagnosed each year in the UK and about half could potentially benefit from the discovery, according to the researchers.
Oestrogen-sensitive breast cancers are commonly treated with drugs, such as Tamoxifen, that block the hormone's receptor.
In future, they could be combined with progesterone to mount a two-pronged attack on the disease, but first, the effectiveness of the approach must be tested in clinical trials.
Dr Emma Smith, senior science communication officer at Cancer Research UK, said: "This exciting study in cells shows how a cheap, safe and widely available drug could potentially improve treatment for around half of all breast cancer patients."
