The data suggest the increase in risk was greater for women diagnosed at a younger age. Hereditary factors are generally more important in cancers that occur in younger women.

Professor Easton adds: "This is perhaps the largest study of its kind. It is the most reliable method for calculating the impact of potential breast cancer genes, and is also being applied to other types of cancer."

Scientists believe the normal CHEK2 gene shuts down cells in a safe and controlled way upon detection of DNA damage. This prevents the cell from passing on faulty DNA, and allows time for repair mechanisms to be engaged. If a normal cell is like a car on the motorway, CHEK2 is responsible for spotting problems, pulling onto the hard shoulder and calling the AA.

CHEK2*1100delC is missing a vital piece “ the brake pedal in the car “ and is unable to initiate shutdown. Carrying this version means faults in other genes are more likely to evade the body's own repair processes and replicate themselves, potentially leading to a tumour.

Professor Robert Souhami, Director of Clinical and External Affairs at Cancer Research UK, says: "Identifying the first of a new set of breast cancer genes puts us in a much better position to tackle breast cancer, both through testing high-risk groups and eventually through new clinical strategies. " cancerresearchuk