HRT helps relieve symptoms of the menopause
New evidence of a link between hormone replacement therapy and a raised risk of breast cancer has been put forward by US researchers.
The New England Journal of Medicine research found breast cancer risk fell sharply when women stopped taking HRT.
A UK expert said a 50% drop in HRT use in recent years had probably stopped up to 1,000 breast cancer cases a year.
But another group of experts said the fall in breast cancer rates may be due to other factors.
The interpretation of the original 2002 "Women's Health Initiative" study, linking combined oestrogen and progestin HRT with breast cancer, has been hotly debated.
This combined version remains the most commonly-prescribed HRT in the UK.
The Californian researchers said that their new findings would end debate over the link.
After the move away from combination HRT in the US after 2002, the number of breast cancer cases fell, a sign to many that the connection between HRT and breast cancer was genuine.
Others argued, however, that a reduction in the frequency of mammograms among women who ditched their HRT might have contributed to the apparent drop.
The latest research not only kept monitoring 15,000 women from the original study, who had all been urged to stop taking HRT in 2002, but looked at data for women not originally involved, who had been given no specific advice on giving up.
In the first group, the incidence of breast cancer was much higher in the hormone group in the five years leading up to 2002, then dropped very rapidly, with the number of diagnoses falling 28% in 12 months.
The women had roughly the same number of mammograms before and after 2002.
Many women in the second group also chose to stop taking the tablets, and this 50% decline in hormone use coincided with a 43% fall in breast cancer rates between 2002 and 2003.
Women in the second group who carried on taking HRT were at higher risk of cancer - with a woman who continued for five years doubling her breast cancer risk every year, the researchers said.
Dr Marcia Stefanik, from Stanford University, said: "This is very strong evidence that oestrogen plus progestin causes breast cancer.
"You start women on hormones and within five years their risk of breast cancer is clearly elevated. You stop the hormones and within one year their risk is essentially back to normal.
"It's reasonably convincing cause-and-effect data."
Professor Valerie Beral, director of the Cancer Research UK Epidemiology Unit at Oxford University said that many studies since 2002 had agreed with the initial finding.
She stressed the "good news" - that breast cancer risk caused by HRT fell soon after women stopped it, rather than persisted for many years.
She said: "In the UK, where there has been a 50% fall in the number of women taking HRT, this means 1,000 fewer women with breast cancer every year."
Cancer Research UK advises that HRT can be taken by women with severe menopausal symptoms for short periods
However, not everyone is persuaded by the latest research findings, with some specialists insisting that HRT remains an important treatment for women whose lives are badly affected by menopausal symptoms.
Dr David Sturdee, president of the International Menopause Society, which represents HRT specialists, said that women should speak to their doctors to see if HRT would be appropriate.
He said: "There's no doubt there has been a drop in breast cancer rates, which is very good news, but this started before the reduction in HRT use.
"Breast cancer takes years to develop, so if this drop was due to stopping HRT, we wouldn't be seeing it just yet.
"There's something happening, which is worth investigating, but it's unlikely to be HRT."
A spokesman for Breakthrough Breast Cancer said previous research had suggested that the extra risk of breast cancer linked to HRT disappeared after five years.
He said the suggestion of a swift return to normality in the research needed to be reproduced in other studies.