The new study, led by experts at the University of Florida, reveals a strong correlation between air pollution and breast density for the first time.

It highlights a specific link between breast risk and the fine air particles that are particularly a problem in the emissions of diesel cars.

Medical experts are increasingly aware of the impact of diesel air pollution fumes on human health, including the risk of asthma, heart disease and dementia, but this is the first time such a strong link has been made to breast cancer.

Fumes and toxins in polluted air are already estimated to contribute to the deaths of 40,000 people in Britain every year - but the new evidence suggests they may also be linked to some of the 11,400 breast cancer deaths recorded annually.

The UK is notoriously bad at controlling air pollution, with 37 cities across Britain persistently breaching legal limits of air toxins.

The researchers, whose work is published in the journal Breast Cancer Research, examined mammogram scans from 279,967 women in the US.

They found that women with high breast density were 19 per cent more likely to live in areas with high levels of fine particle pollution.

For every one unit increase in the particles - known as PM2.5 - a woman's chance of having dense breasts went up 4 per cent.

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