The majority of breast cancers in women with a family history of the disease occur after the age of 50.

Professor Val Beral

They analysed the combined results of 52 separate studies into the risk of developing breast cancer.

In total, they examined 58,209 women with breast cancer and 101,986 women who had not developed the disease.

Relative risk

They found that 7.8% of women with one close relative who had developed breast cancer went on to develop the disease themselves.

For women with two affected relatives the figure was 13.3%, and for women with three affected relatives it rose to 21.1%.

The number of affected relatives is more important than the age the relatives were when breast cancer was diagnosed.

For example, for women with one affected first degree relative, the lifetime risk is 12.3% if the relative was aged more than 60 at diagnosis, and 16.1% if the relative was aged less than 40.

The researchers stress that eight of nine women who develop breast cancer do not have an affected mother, sister, or daughter.

They also stress that although women who have a history of breast cancer are at increased risk of the disease, most will never develop breast cancer, and those who do will probably be over 50 when their cancer is diagnosed.

Lead researcher Professor Valerie Beral, Head of Imperial Cancer's Cancer Epidemiology Unit, said: "Contrary to popular belief, the majority of breast cancers in women with a family history of the disease occur after the age of 50 rather than at younger ages.

"It is therefore important that women with a family history remain vigilant throughout their lives and ensure they attend for screening when they are invited to do so."

Mistaken perception

This will go some way towards allaying unwarranted fears.

Dr Kate Law

Dr Kate Law, the Cancer Research Campaign's Director of Clinical Trials, said: "Breast cancer has become the most talked-about and publicised cancer, and with this awareness has come a mistaken perception that this is a disease which mainly affects young women.

"This very important piece of research will go some way towards giving women a realistic idea of their breast cancer risk and allaying unwarranted fears.

"It is particularly significant because it is such a large study, comparing women who develop breast cancer with those who do not.

"A finding which may come as a surprise is that the majority of breast cancers in women with a family history of the disease occur after the age of 50, rather than at a younger age."

Dr Law said that all women should regularly examine their breasts, and report any changes without delay.

The research is published in The Lancet medical journal.