White women are more likely to develop cancer than their South Asian and black counterparts because of "lifestyle and reproductive factors", a new study suggests.
Compared to women from these ethnic backgrounds, white women in the England have higher breast cancer rates, researchers said.
But when taking into account risk factors such as breastfeeding rates and the number of children women had, the risk of developing breast cancer was found to be similar for women of all ethnic groups.
Researchers from the University of Oxford and Oxford University NHS Trust examined data from more than 1,000,000 women in England who participated in the Million Women study.
Women aged 50 to 64 were enrolled into the study, designed to investigate links between health and lifestyle, from 1996 to 2001. Participants completed questionnaires about living habits, medical and social factors and cancer data was obtained from NHS cancer registries.
The authors found that after around 12 years, 217 of 5,877 South Asian women developed breast cancer, as did 180 of 4,919 black women and 45,191 of 1,038,144 white women - meaning South Asian women had an 18% lower rate of breast cancer compared with white women, and black women had a 15% lower rate compared to white women.
The study, published in the British Journal of Cancer, also found numerous differences in known risk factors such as alcohol consumption and use of menopausal hormone therapy, among others.
For instance South Asian and black women had more children than white women and were more likely to breastfeed them - 69% of white women said they had breastfed their children compared to 83% of black women and 85% of South Asian women.
Meanwhile, 75% of South Asian women said they were non-drinkers compared to 38% of black women and 23% of white women.
And 35% of white women said they were a current user of menopausal hormone therapy compared to 22% of South Asian women and 29% of black women.
Women from black and South Asian backgrounds were also less likely to have a first degree relative with breast cancer.
After the researchers excluded these, and other lifestyle and reproductive factors, from the analysis the risk of developing breast cancer was found to be similar for women of all ethnic groups.
They concluded that the differences "largely, if not wholly" account for the different rates of breast cancer among women from different ethnicities.
The authors wrote: "These findings indicate that the lower incidence rates of breast cancer seen in South Asian and black women as compared with white women in England are largely, if not wholly, because of differences in known risk factors for the disease.
"Once adjustments were made for risk factors including age at menarche, height, childbearing and breastfeeding history, alcohol consumption, and use of menopausal hormone therapy, South Asian and black women were shown to have similar breast cancer risks to white women."
They added that many of the black and South Asian women in the study were first-generation immigrants and warned that as second and subsequent generations of women of ethnic minority origin change their lifestyles, their risk of breast cancer will increase.
Study author Dr Toral Gathani, from the University of Oxford, said: "In this study of largely post-menopausal women in England, we see that the lower risk of breast cancer in South Asian and black women is largely explained by differences in lifestyle and reproductive patterns.
"It's important for women of all ethnic groups to understand what are the modifiable risk factors for breast cancer, such as obesity and excessive alcohol consumption, and to take measures to reduce their risk."
Dr Julie Sharp, Cancer Research UK's head of health information, added: "Women can reduce their risk of breast cancer by cutting down on alcohol, keeping a healthy weight by eating a balanced diet and by keeping active.
"If women notice any changes to their breast such as lumps, any skin or nipple changes, or changes in their size, shape or feel they should tell their doctor straight away. It's probably not cancer, but if it is, getting it diagnosed as early as possible gives the best chance of survival."