Smokers in England and Wales can now be fined £50 for lighting up in a car when children are present.

The new law, which comes eight years after smoking was outlawed in public places, is designed to protect under-18s from the effects of passive smoking.

Researchers in Newcastle found children in cars where someone was smoking were exposed to higher concentrations of carcinogenic chemicals than previously thought.

In tests, they found a driver smoking with the window open created air pollution levels in the back of the car that were 100 times higher than the recommended safety guidelines.

Newcastle University’s Dr Anil Namdeo, who led the research, said the levels of carcinogens peaked within minutes of the driver lighting a cigarette.

“We saw a rapid increase in the levels of these harmful chemicals, fine particles known as PM2.5, not just around the driver but also around the child’s car seat,” he said.

“With the window closed the levels peaked at several hundred times the safe limit but even with the window open we saw a significant rise to well above the safe recommended limits.”

Professor Dame Sally Davies, the Chief Medical Officer, described the legislation as a landmark in protecting children.

“Smoking just a single cigarette in a vehicle exposes children to high levels of air pollutants and cancer-causing chemicals like arsenic, formaldehyde and tar, and people often wrongly assume that opening a window, or letting in fresh air, will lessen the damage,” she said.

However, police forces are unlikely to fine many drivers, with the National Police Chiefs’ Council saying officers will “advise” drivers of the new law.

“As the existing smoke free law extends to vehicles, police forces will be taking an educational, advisory and non-confrontational approach when enforcing the new legislation,” said a spokeswoman.

“This would see people being given warnings rather than being issued with fines, which would give time for public awareness of the offences to build.”