Pollution and air quality hot-spots in many British cities have become so alarmingly high, it is almost as damaging as mothers who smoked during pregnancy, researchers in Edinburgh and Aberdeen Universities have discovered.
Scientists gathered data on foetal growth from ultrasound scans and maternity records involving almost 14,000 pregnancies in Scotland over a nine year period.
Lifestyle factors, such as smoking, were considered alongside air quality in the area.
Dr. Tom Clemens, who led the study, has urged the World Health Organization and EU to urgently review their definition of "acceptable" emissions levels amid concern they remain too high.
"Our findings suggest that there may not be a truly 'safe' level of exposure during pregnancy. A foetus with a non-smoking mother exposed to high pollution levels is only slightly better off than one with a smoking mother exposed to low levels of pollution.
"This implies that the effect of exposure to the highest levels may be almost as bad as smoking," Dr. Clemens said.
The study, published in the scientific journal Environmental International, has ignited calls to bring forward the UK Government ban.
It has already been estimated two thirds of the British population are living in towns and cities where nitrogen dioxide pollution breaches the lawful level of 40 micrograms per cubic metre of air.
British ministers have already pledged to outlaw diesel and petrol cars by 2040 in a bid to tackle growing environmental risks to public health.
"Since the 2008 Beijing Olympics, outdoor air pollution has been recognised as a problem and the evidence we need to tackle it continues to pile up. It is time to do something about it," warned Dr. Steve Turner, a co-author of the study from the University of Aberdeen.
In February, the European Commission admitted air quality laws had been flouted in more than 130 cities across 23 of the 28 EU member states, including the UK.
Poor air quality in Scotland is already estimated to reduce average life expectancy by three to four months compared with seven months in England and Wales.