Thousands of cases could be avoided if more mothers persevered feeding their babies breast milk for six months, experts say.
Research involving almost 37,000 women found those who had breastfed were 10% less likely to get the disease. And they were 20% less likely to develop one of the worst forms of breast cancer, known as triple negative.
In Britain, just 23% of women breastfeed for six months, often giving their babies some formula milk as a supplement. But the World Health Organisation recommends exclusive breastfeeding for six months.
US researchers at two charities and Washington University in St Louis, Missouri, analysed 27 studies covering 36,881 women. Those who had breastfed for any length of time reduced their risk of breast cancer by 10%.
Crucially, they were also a fifth, or 20%, less likely to get triple negative cancer, which makes up 15 to 20% of cases and is very hard to cure. It is called triple negative because it does not respond to any of three powerful treatments.
Breastfeeding lowers levels of the hormone oestrogen, which can trigger cancer. Some scientists also believe the process of producing milk stops cancer cells forming.
The researchers said nursing for three months would “significantly reduce the risk” of breast cancer.
Dr Graham Colditz presented the findings at the San Antonio Breast Cancer Symposium in Texas. He said: “Importantly, breastfeeding was clearly protective for triple negative breast cancer – the subset of this disease for which we have limited treatment options.”
He called for “greater public health support” for breastfeeding.
NHS figures show 82% of mothers start to breastfeed, but only 55% do it for six weeks and just one in 100 carries on with no formula milk for six months.
Commenting on the research, Jacque Gerrard of the Royal College of Midwives, said: “There is now ‘gold standard’ evidence which underpins the benefits of breastfeeding in terms of health outcomes for children and young people. Breastfeeding starts with skin-to-skin contact at birth, now normal practice in the UK, and provides a unique opportunity for mother and baby attachment.
“It is without doubt a positive way for a woman to give her newborn the best possible start in life, and support her own health at the same time.”