The protection was more pronounced for aggressive tumours. The preventive effect was not however found in women developing breast cancer after the menopause, which occurs around the age of 50.

About 8,000 women develop pre-menopausal breast cancer each year.

The latest study adds to evidence that vitamin D may play an important role in preventing breast and other cancers ñ particularly when combined with calcium.

Researchers at Harvard Medical School in Boston and its affiliate, Brigham and Women's Hospital, assessed the diets of more than 10,000 pre-menopausal and 21,000 post-menopausal women. When the study started in the early 1990s, the women filled out a food survey and a questionnaire about their medical history and lifestyle.

The women completed regular questionnaires over the following 10 years.

Altogether 276 pre-menopausal women and 743 postmenopausal women developed the disease.

Among pre-menopausal women, those in the group with highest intake of calcium -- more than 1,300mg a day -- had a 40% lower risk of disease.

Those in the group with the highest vitamin D intake -- around 550 international units a day -- had a 35% lower risk.

The link was found to be strongest for fastergrowing tumours, which are more often diagnosed in younger women.

It did not find any change in risk conferred by eating the nutrients after the menopause. The study included food and supplements as sources of calcium and vitamin D but did not look at sun exposure.

Jennifer Hsiang-Ling Lin, who led the study, said, "I would recommend women, especially premenopausal women, have adequate intakes of calcium and vitamin D to keep up their overall health, and additionally, to potentially reduce risk for developing breast cancer."

Vitamin D is found in salmon, tuna and other oily fish, and is routinely added to milk. However, diet accounts for very little of the nutrient that actually makes it into the bloodstream, with most coming from sun exposure.

A 3.5oz serving of oily fish provides almost 400 international units of the vitamin, two eggs provide 80 and a glass of milk 100.

Good sources of calcium include milk, yoghurt, green vegetables, bread and nuts.

A 3.5oz serving of cheddar cheese provides 700milligrams of calcium, a large portion of cooked broccoli 170mg, six slices of white bread 170mg and a serving of dried figs 100mg.

The study is published today in the Archives of Internal Medicine.

From the June 4, 2007, Prepared Foods e-Flash