US researchers discovered there was a 28% reduction in risk of breast cancer within one year of stopping HRT treatment with oestrogen and progestin.
The results show women who spend at least five years on HRT double their risk of getting the disease each year.
A previous study by the Women's Health Initiative (WHI) found links between HRT and cancer, blood clots and strokes. This study prompted a 40 million drop in prescriptions for the treatment in the US in 2005.
However, it was argued that the reduction in breast cancer cases was a result of improved mammogram screening.
But Professor Marcia Stefanick, from Stanford University in California, who co-authored the new study, said this new study will confirm that oestrogen plus progestin causes breast cancer.
She said: 'You start women on hormones and within five years, their risk for 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.'