However, the new laboratory tests are the first time researchers have witnessed their endocrine-disrupting qualities at work in this way.

The team in charge of the experiments have now called for better regulation of substances containing the essential oils and for further studies into the long-term effects of exposure.

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Gynaecomastia is most common in teenage boys and older men, symptoms varying from a small amount of extra tissue around the nipples to more prominent breasts.

The condition can cause breast tissue to be tender or painful.

A growing number of reported cases of gynaecomastia have coincided with topical exposure to the oils, with symptoms subsiding after the subjects stopped using the products.

Jeffrey T. Ramsey, who led the research at the National Institute of Environmental Health Sciences in North Carolina, said: “Our society deems essential oil as safe.