A blood test to identify women at high risk of breast cancer could be available in five years, say scientists.
Scientists at Imperial College London have identified a 'genetic switch', carried by one in five women, which doubles their risk of developing breast cancer.
Experts described the breakthrough discovery as "exciting" and said signs of the disease could be detected "many decades in advance".
Dr James Flanagan, who led the new research, said the test could be available in five to ten years, the Telegraph reported.
The 'genetic switch' is influenced by lifestyle factors such as alcohol, smoking, pollution, and hormones including HRT.
Carrying the genetic alterations increase a woman's lifetime risk of developing breast cancer from one in eight in the general population to one in four.
These tiny genetic changes could be detected in blood samples years before symptoms of breast cancer developed.
Scientists analysed blood samples from 1,380 women of various ages, 640 of whom went on to develop breast cancer.
On average, the blood tests were carried out three years before diagnosis. In some cases they pre-dated the discovery of breast cancer by up to 11 years.
The results were especially clear in blood samples from women under the age of 60.
The changes are also associated with lymphoma and leukaemia meaning the test could have implications in other cancers.
A strong association was found between molecular changes in a white blood cell gene called ATM and breast cancer risk.
"We are working towards prevention. If we can identify women at high risk of cancer we can work towards preventing it and could reduce the incidence of breast cancer quite dramatically," said Dr Flanagan.
"We have found one marker, we need to work towards finding them all and then we will have a more useful test," he stated.
Baroness Delyth Morgan, chief executive of Breast Cancer Campaign, said: "Dr Flanagan's research into epigenetics is so exciting because it suggests that there is every possibility the risk of developing breast cancer could be decided many decades in advance.
"By piecing together how this happens, we can look at ways of preventing the disease and detecting it earlier to give people the best possible chance of survival," Delyth Morgan added.
The latest findings are published in the journal Cancer Research.