A major study into the long-term effects of hormone replacement therapy involving more than 16,000 women has been halted early because the risks so clearly outweighed the benefits.

Researchers in the US were forced to abandon the eight-year investigation after it had been running five years.

The decision was taken after it was found that the HRT treatment increased the risk of invasive breast cancer by 26 per cent, of heart attacks and other coronary ailments by 29 per cent, and of strokes by 41 per cent.

Women receiving HRT also had double the risk of blood clots compared with those unknowingly taking dummy pills during the trial.

Overall, cardiovascular risk was increased by 22 per cent in the HRT group.

The benefits included a 37 per cent reduction in rates of bowel cancer, while hip fracture rates were reduced by a third and total fractures by 24 per cent.

A total of 16,608 post-menopausal women, aged 50 to 79, took part in the trial at 40 US centres.

Half were given a daily tablet containing a combination of the female hormones oestrogen and progestin, a version of progesterone. The rest had a matching placebo pill.

Millions of women all over the world take this type of HRT, which combines two hormones in a single pill.

The breast cancer finding was the decisive factor in ending the study early and releasing the results on the website of the Journal of the American Medical Association.

Dr Suzanne Fletcher and two colleagues, from Harvard Medical School in Boston, said: "The whole purpose of healthy women taking long-term oestrogen/progestin therapy is to preserve health and prevent disease.

"The results of this study provide strong evidence that the opposite is happening for important aspects of women's health, even if the absolute risk is low."

Six million HRT prescriptions were issued in England last year.

A spokesman for the Medicines Control Agency said: "UK product information for HRT already contains extensive warnings about the risk of breast cancer."

When the data from the US study became available the agency would rapidly assess it, he added.

Malcolm Whitehead, director of the Amarant Trust which provides help and advice to menopausal women in the UK, said the picture was far from clear.

It was important to remember that not every type of HRT had the same effect, but he warned against long-term HRT use