In a small study of women with the illness they found that patients with early breast cancer had higher levels of the vitamin than those with more advanced disease.

"Vitamin D levels are lower in women with advanced breast cancer than in early breast cancer," said Dr Carlo Palmieri, of Imperial College London.

"It lends support to the idea that vitamin D has a role in the progression of breast cancer," he told Reuters.

The researchers measured the amount of vitamin D in blood samples from 279 women with breast cancer. In 204 women, the disease was in its early stages. It was more advanced in the other 75.

"We know from previous studies that breast cancer incidence is higher in women who live in higher latitudes and have less sun exposure," said Palmieri.

Vitamin D, the so-called sunshine vitamin, is found in fortified milk and dairy products, cod liver oil and some fatty fish.

The body produces vitamin D in the skin when it is exposed to sunlight. Laboratory tests have also shown that vitamin D can stop cancer cells from dividing.

Breast cancer is the most common cancer in women, with more than a million cases detected worldwide each year, according to the International Agency for Research on Cancer (IARC) in Lyon, France. The earlier the cancer is detected and treated, the greater the chances of survival.

Although the scientists do not know whether the low levels of vitamin D are a cause or a consequence of the cancer, their findings and the results of other studies suggest that low levels of vitamin D may be involved in the progression of early breast cancer to more advanced stages.

"The next thing in this research is to try and understand the potential causes and mechanisms underlying these differences and the precise consequences at a molecular level," said Palmieri, who reported the finding in the Journal of Clinical Pathology.

"We also need to look at the potential clinical implications of monitoring and maintaining high circulating vitamin D levels in breast cancer patients," he added in a statement.

Breast cancer is treated with surgery and radiotherapy, chemotherapy and hormone treatment, or a combination of them, depending on the cancer and stage of the illness.

Factors which can increase a woman's risk of breast cancer include having a mother or close relative with the disease, inherited mutations in the BRCA1 or BRCA2 genes, an early puberty, late menopause and not having any children.