The risk is greatest with the most common form of HRT, estrogen plus progestogen, if it's taken for five years, say researchers from Oxford University. They estimate that one in 20 cases of breast cancer in the UK are caused by the drug, designed to help women cope with the worst symptoms of the menopause.
Women taking the estrogen-only form had a slightly lower risk, with one in 70 developing breast cancer.
And women need to know the risks persist for 10 years afterwards, the researchers say, and they don't disappear the moment they stop taking HRT, which has been the common perception.
HRT prescribing has been steadily rising over the last 10 years or so, with some studies downplaying health risks, but the Oxford researchers say it is vital to turn back the dial to the early 1990s when dangers of the drug were first discovered.
The researchers trawled health databases to identify 108,000 women, with an average age of 65, who had developed breast cancer; of those, 51 percent had taken HRT or MHT (menopausal hormone therapy) for an average of seven years.
The average risk of breast cancer in a woman aged between 50 and 69 who hasn't taken HRT is 6.3 percent, but this rises to 8.3 percent in women who have been taking HRT for five years. The risk increases further if it's taken for 10 years or more.