The drug targets the hormone oestrogen
Campaigners have called for a fast-track assessment of a new breast cancer drug which may help save lives.
A global study found Femara (letrozole) was of most benefit to women at highest risk of the cancer returning, the New England Journal of Medicine reported.
In women whose cancer had spread to the lymph nodes, the drug cut the risk of a recurrence of disease by 29% compared to 'gold standard' tamoxifen treatment.
The study involved 8,000 women, including more than 400 from the UK.
The findings contributed to a decision earlier this month by the Medicines and Healthcare products Regulatory Agency (MHRA) to approve a licence for the use of the drug for postmenopausal women with early breast cancer straight after surgery.
However, the NHS drug watchdog is not due to rule on whether the drug should be made widely available until late next year.
The study, co-ordinated by the Senology Center of Eastern Switzerland in Kantonsspital, looked at which drug women in the study were initially treated with.
They found Femara cut the risk of disease returning in women who had already undergone chemotherapy by 28%, compared to tamoxifen therapy.
Among all women taking Femara there was a 27% reduction in the risk of cancer spreading to other parts of the body compared to those taking tamoxifen, and a 19% drop in the risk of breast cancer returning.
Femara is a type of drug called an aromatase inhibitor which stops the natural production of oestrogen - the hormone that is responsible for the growth and recurrence of many breast cancers.
There has been concern recently that cancer patients in some areas are being denied access to the newest drugs because of financial concerns of NHS trusts.
This has led some to take legal action to make their health authority fund the treatment.
The National Institute for Health and Clinical Excellence (NICE) is due to rule on the use of aromatase inhibitors by the NHS next autumn. However, pressure is building for the process to be speeded up.
The drug has already been approved in Scotland for post-menopausal patients - but only after they have been treated with tamoxifen.
Nigel Bundred, a professor in surgical oncology at Wythenshawe Hospital, Manchester, said: "These results are fantastic news and give hope to women with breast cancer - especially those at high risk of their cancer recurring.
"They show that Femara is more effective than tamoxifen when given to women after surgery and offers even greater advantages to these particularly vulnerable women."
Liz Caroll, from charity Breast Cancer Care, said: "These results further suggest the benefits of using aromatase inhibitors over tamoxifen in treating early invasive breast cancer and indicate that many more lives could be saved.
"Many women, like those we support, with breast cancer will welcome the news that they might benefit from this new treatment option, as will the healthcare professionals treating them."
However, Professor Karol Sikora, an expert in cancer medicine at Hammersmith Hospital, London, said: "What we need is to see a really slick version of NICE that can approve a drug within two days.
"There is going to be no more data when they finally approve it than they have got now - and that is the tragedy."
Treatment with tamoxifen was associated with higher rates of blood clots, endometrial cancer, and vaginal bleeding.
On the other hand, Femara therapy was tied to higher rates of skeletal and heart problems, as well as high cholesterol levels.