A study linked low levels of the chemicals to the development of vulnerable and hormone-sensitive breast tissue in mice.

Experts believe the findings have alarming implications for human health.

The mammary glands of pubescent female mice developed structurally in a way that made them more likely to develop breast cancer. They also became unusually sensitized to estrogen, which fuels the vast majority of breast tumors in humans.

Chemical Leaches out of Products

The research focused on bisphenol-A (BPA), a compound used in large quantities in the manufacture plastic food containers, the resins that line food cans, and dental sealants.

Research has shown the chemical leaches out of products and may be absorbed at low concentrations into the body.

Scientists working with animals had previously shown that BPA is potentially damaging to health, but this new study is the first to suggest that even extremely weak levels of exposure in the womb may be harmful.

A team of US researchers, led by Professor Ana Soto, from Tufts University School of Medicine in Boston, Massachusetts, administered tiny doses of BPA to groups of pregnant mice.

The doses, 25 and 250 nanograms -- a billionth of a gram -- of the chemical per kilogram of body weight, were designed to mimic the levels humans are likely to be exposed to.

The mice were treated during the latter half of their pregnancies, and for about four days after giving birth. Their offspring were then examined at 30-days-old when they reached puberty.

Sensitivity to Estrogen

One of the most striking effects of BPA exposure was a large increase in the number and density of terminal end buds, part of the mammary gland's milk-producing structure. It is here that breast tumors typically form.

Scientists also observed a significant drop in the number of cells in this region marked out for death -- or apoptosis -- a natural defense mechanism that weeds out damaged cells that may turn cancerous.

Researchers also found animals previously exposed to a higher dose of BPA developed mammary glands that were more sensitive to estrogen.

A number of mice had their ovaries removed, to prevent them producing the hormone, and were then exposed to an artificial estrogen source. Their sensitivity to estrogen was revealed by the degree to which it affected the further growth and structure of terminal end buds.

Susceptibility to Breast Cancer

Researchers, reporting in the latest issue of Endocrinology, wrote: "These correlations suggest that perinatal (shortly before or after birth) exposure to BPA in particular, and to estrogens in general, may increase susceptibility to breast cancer."

Every year, about 40,000 women are diagnosed with breast cancer in Britain, and nearly 13,000 die from the disease. While survival rates have improved, the number of cases in Britain and Western countries is rising.

Professor Frederick vom Saal, a reproductive biologist from University of Missouri-Columbia in the US, said it could be assumed every member of the population was "chronically exposed" to BPA.

"This is of tremendous concern, because this is clearly a study that's relevant to human exposure levels to this chemical," he said.

"What she (Professor Soto) is showing, which is consistent with a number of other reports, is that the mammary gland is being altered in its response to sex hormones. This is a chemical that is not only structurally altering the organ, but is permanently changing the function of the organ, and that is of huge public concern."