Long-term use of combination hormone replacement therapy (HRT) can double the risk of developing breast cancer, a study of more than a million women has found.

The research also shows that women taking oestrogen-only HRT increase their risk by 30 per cent. The study, conducted by Cancer Research UK's epidemiology unit in Oxford, of 1.1 million British women's medical histories, concluded that HRT use was responsible for 2,000 cases of breast cancer a year in Britain. Every year 41,000 women are diagnosed with breast cancer and about 13,000 die.

The type of HRT that appeared to carry the highest risk, which contains the hormones oestrogen and progestogen, became popular after oestrogen-only therapies were found to increase the chance of getting cancer of the womb lining.

A total of 1.5 million women in Britain take HRT, of which 750,000 take the combined therapy. The study found that the increased risk of breast cancer that 10 years' use of combination HRT brings was comparable to having a mother and sister with breast cancer. A woman's risk of developing breast cancer decreases when she stops HRT and returns to normal levels five years after cessation, according to the research.

Dr John Toy, medical director of Cancer Research UK, which funded the study, said: "A woman wanting to take HRT for a long time would be extremely wise to consider carefully the findings of this large study and other relevant research."

Most menopausal women who use HRT, about 98 per cent, will not get breast cancer as a result of it. HRT is effective at dealing with the symptoms of the menopause and is also licensed to prevent osteoporosis. Osteoporosis can be a life threatening condition - 25 out of every 100 women who suffer hip fractures die within six months and, of the survivors, 50 per cent will not regain their independence.

Professor Aidan Halligan, the Deputy Chief Medical Officer for England, said that women should not suddenly stop taking HRT and has announced that the Department of Health would commission an analysis of the risks and benefits of the treatment. The Committee on the Safety of Medicines has reviewed the data and written to all health professionals. They stressed short-term HRT use is still beneficial, but those taking it for more than a year should heed the risks and discuss them with their GP.