Sooty air pollution in towns and cities increases the chances of women giving birth to small babies, new research has shown.

A study involving millions of births around the world found that higher pollution levels raised the risk of low birth-weight.

Although small, the effect is said to be statistically significant. At national population scales it could have an important impact on child health, said the researchers.

Babies are underweight at birth if they tip the scales at less than 2.5kgs, or 5lbs 8oz.

They face an increased risk of dying in infancy, as well as chronic poor health and impaired mental development.

The new study, the largest of its kind ever conducted, focused on tiny sooty carbon particles called PM10s and even smaller PM2.5s which are known to be linked to heart and lung problems and early death.

They originate from a number of sources, including diesel exhausts and the chimneys of coal-fired power stations and factories.

Professor Tanja Pless-Mulloli, who led the UK arm of the study at the Newcastle University, said: "As air pollution increases we can see that more babies are smaller at birth, which in turn puts them at risk of poor health later in life.

"These microscopic particles, five times smaller than the width of a human hair, are part of the air we breathe every day. What we have shown definitively is that these levels are already having an effect on pregnant mothers."

The research, published in the journal Environmental Health Perspectives, examined the impact of a 10 microgram per cubic metre increase in average exposure to pollution particles over the course of a pregnancy.

For PM10s, this raised the chances of having a low birth-weight baby by 0.03%, which was said to be statistically significant. In the case of PM2.5s, a much larger 10% risk increase was seen.

The research also showed a continual trend of elevated low birth-weight risk with higher levels of air pollution.

Pless-Mulloli added: "The particles which are affecting pregnant mothers mainly come from the burning of fossil fuels. In the past the culprit may have been coal fires, now it is primarily vehicle fumes.

"Currently in some parts of London we see around 40 units of particulate air pollution and in Newcastle it is around 20 units but going back to the 1960s we saw around 700 units of air pollution. While much has been done to improve air quality, this study shows we can't be complacent as we've shown that clean air is really important for the health of our newborns."

The scientists collected data on more than 3m births at 14 locations in the UK, north and south America, Asia and Australia.

They concluded: "The estimated combined associations, although relatively small, could be of major public health importance considering the ubiquitous nature of particulate air pollution exposure, and therefore the potential for considerable population attributable risk, particularly given evidence of perinatal (around the time of birth) and life-long effects of LBW (low birth weight) on health."

Dr Tony Fletcher, senior lecturer in environmental epidemiology at the London School of Hygiene and Tropical Medicine, said: "This new study is very helpful in establishing another health impact of air pollution. Because the average effect is quite small, it needs enormous multi-country studies such as these to quantify the effect.

"The study is of excellent quality and the conclusions are clear: while the average effect on each baby is small and so should not alarm individual prospective parents, for the whole population these small risks add up across millions of people. Another reason for London and other large cities to force traffic-related pollution down to lower levels."