Researchers from Stanford University in California say the findings strengthen the hotly debated link between HRT and breast cancer, particularly in women who stay on the treatment for more than five years.

The results of the study, which involved more than 57,000 US women, mirrors the decline in breast cancer rates in Australian women over 50 since 2002.

In that year, research showed women taking estrogen and progestin together had higher rates of breast cancer and heart disease. The study led to a sharp drop in HRT prescriptions in Australia, the US and Britain.

Karen Canfell, an epidemiologist at the Cancer Council NSW, said breast cancer rates in Australia fell by 7 per cent from 2001 to 2003.

"The good news is that this new study adds to the large body of evidence that when women stop taking HRT their risk of breast cancer quickly goes back to normal," Dr Canfell said.

The findings, published in the New England Journal Of Medicine , show that women who stay on the therapy for at least five years double their risk of developing cancer for every year that they are on the treatment.

But stopping therapy quickly negated that risk, suggesting that hormone withdrawal led to a regression of preclinical cancers, researchers said.

Twelve months after participants in the women's health initiative trial stopped HRT treatment, breast cancer cases fell by 28 per cent.

The percentage of patients who went for mammogram tests during that time remained stable, meaning that the fall in the number of cases was not because they were being missed by doctors, the researchers said.

Last year an international panel of experts said that using HRT was safe for most women going through menopause.

Although combined forms of HRT may slightly increase the chances of breast cancer, the effect is dwarfed by other risk factors, such as obesity, diet and alcohol, the review from the First Global Summit on Menopause-Related Issues said.

Associate Professor Emily Banks, of the National Centre for Epidemiology and Population Health at the Australian National University, said the large drop in HRT use meant about 600 fewer Australian women developed breast cancer every year.

Dr Helen Zorbas, the chief executive of the National Breast and Ovarian Cancer Centre, said combined HRT should only be used as a short-term option for the control of severe menopausal symptoms.

She said doctors should weigh up the risks before they prescribe HRT, used to ease the often debilitating symptoms of the menopause, which can include hot flushes, nausea and insomnia.