This is the first time the risk for women who do not have a faulty BRCA gene but have one first-degree relative under 50 with breast cancer and at least one other relative with breast cancer, or three relatives of any age, has been quantified.

The lifetime risk of breast cancer in the general population is one in nine. For the group studied, this risk increased to just over one in three.

The researchers believe that a significant family history of breast cancer alone could be strong enough grounds for doctors to offer preventative treatments - such as tamoxifen, which is given to most women with breast cancer to help prevent the disease from returning.

Lead researcher, Dr Steven Narod, based at the University of Toronto in Canada, said: "Although the risk of breast cancer for this group of women - those with a family history of the disease but who don't carry a faulty BRCA gene - is not as high as those who do carry the gene, it is still significant enough for doctors to start thinking about appropriate preventative treatments to stop women developing the disease.

"This is the first time the breast cancer risk for this group of women has been measured, and it's significantly higher than that of the general population. It's important to start thinking about action to prevent breast cancer in women who are at high risk of developing the disease."

Currently in the UK, women who have a strong family history of breast cancer will be offered annual MRI scans from the age of 40 and will sometimes elect to have preventative surgery.

Previous Cancer Research UK trials have shown that tamoxifen reduces the risk of oestrogen receptor positive breast cancer - the most common kind - by between 30 to 40 per cent in women at high risk of the disease.

Professor Jack Cuzick, Cancer Research UK's epidemiologist who leads on clinical trials to prevent breast cancer, said: "Looking at ways to prevent breast cancer is an exciting field, particularly in those who are at an increased risk of the disease. We're also learning more about which women will respond to preventative treatments, which takes us ever closer to tailoring cancer treatment to each patient. Around 15 per cent of women with breast cancer have a family history of the disease, so around 7,000 women a year in the UK could benefit from this research."

Dr Lesley Walker, director of cancer information at Cancer Research UK, said: "Cancer Research UK has led the way in improving our understanding of the genetics of cancer and identifying those people who would benefit most from interventions to prevent the disease before it develops or if cancer does develop to diagnose it early. These findings will help to strengthen advice to women with a strong family history of breast cancer but no evidence of a BRCA mutation."

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