One genetic region they identified increases breast cancer risk by about 12 per cent in women who carry one faulty copy of the gene and by 23 per cent if they carry two faulty copies.

Interestingly, the other region reduces risk by about 4 per cent with one faulty copy and by 11 per cent with two faulty copies. In the majority of cases, faulty genes increase cancer risk, but in some cases a mutation decreases risk.

This study brings the total number of known genetic regions with common genetic changes that alter breast cancer risk to 13 - eight of which were found by Cancer Research UK funded studies.

The increased risks acquired through these genetic faults are small but as more of these 'low risk' genes are found, it will be possible to create tests for a combination of genes that together significantly increase risk.

This could help doctors make decisions about prevention, diagnosis and treatment for women who are more likely to get breast cancer.

Professor Doug Easton, lead author and director of Cancer Research UK's Genetic Epidemiology Unit at the University of Cambridge, said: "These two new genes bring us closer to developing a better test to identify women who are at a high risk of developing breast cancer, but there are still many more pieces of the genetic puzzle to find."

Scientists in this study scanned the entire genetic code in over 400 women with breast cancer for faulty regions of the gene that cropped up more regularly in cancer patients than in healthy women.

They then tested the most promising regions in over 40,000 women with breast cancer, and 40,000 women without breast cancer, in an international collaboration involving more than 100 scientists from 16 countries.

One in five women in the general population are thought to carry two faulty copies of the markers found in this study to increase risk - rs4973768. This lies on chromosome 3, close to two genes called NEK10 and SLC4A7.

One in 14 women are thought to carry two faulty copies of the marker that decreases risk - rs6504950. This lies on chromosome 17, close to a gene called COX11.

Breast cancer is the most common cancer in the UK with more than 45,500 new cases diagnosed each year.

Lifestyle factors play an important part in influencing the risk of breast cancers, but inherited factors are also important in determining an individual woman's risk of the disease.

Dr Lesley Walker, director of cancer information at Cancer Research UK, said: "This study brings us a step closer to creating a powerful genetic test for breast cancer.

"If we can identify women who are more likely to get the disease and if we can work out how high this risk is, doctors can make informed decisions about how to stop them getting breast cancer in the first place.

"Cancer Research UK would recommend that women worried about their risk of cancer visit their GP."

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