Breastfeeding has a protective effect, especially with particular types of tumour, including the most common hormone-sensitive strain, researchers say.

Women with breast cancer who breastfed their babies are significantly less at risk of the disease recurring or killing them, a study has found.

Overall a history of breastfeeding reduced the chances of cancer returning after treatment by 30 per cent, while the risk of dying was lowered by 28 per cent.

Researchers analysed data on 1636 women with breast cancer who completed a questionnaire about breastfeeding.

They discovered that breastfeeding had a clear protective effect, especially in relation to particular types of tumour, including the most common hormone-sensitive strain.

Lead researcher Dr Marilyn Kwan, from US healthcare provider Kaiser Permanente, said: "This is the first study we're aware of that examined the role of breastfeeding history in cancer recurrence, and by tumour subtype.

"Women who breastfeed are more likely to get the luminal A subtype of breast cancer, which is less aggressive, and breastfeeding may set up a molecular environment that makes the tumour more responsive to anti-oestrogen therapy."

The protection was strongest for women who had a history of breastfeeding for six months or longer.

Luminal A breast cancer includes oestrogen-positive tumours which are driven by the female hormone and are the most commonly diagnosed form of the disease.

These tumours are less likely to spread to other parts of the body than other types and are treatable with hormonal drugs such as tamoxifen and aromatase inhibitors.

Why women who breastfeed their babies develop less aggressive tumours is not entirely clear.

Co-author Dr Bette Caan, also from Kaiser Permanente, said: "Breastfeeding may increase the maturation of ductal cells in the breast, making them less susceptible to carcinogens or facilitate the excretion of carcinogens, and lead to slower growing tumours."

The research appears in the Journal of the National Cancer Institute in the US.