From October 1 drivers in England will be fined $100 if they are caught smoking with children in the car.
The law change is being hailed as “tremendous victory” by The British Lung Foundation.
But UK smokers’ group Forest say the legislation isn’t unenforceable.
“The overwhelming majority of smokers know it’s inconsiderate to smoke in a car with children and they don’t do it,” Simon Clark director Forest told BBC.
“They don’t need the state micro-managing their lives.
“The police won’t be able to enforce the law on their own so the government will need a small army of snoopers to report people.”
However, Public Health Minister Jane Ellison says the new law is an important step forward in protecting children from secondhand smoke.
“Three million children are exposed to secondhand smoke in cars, putting their health at risk.
“We know that many of them feel embarrassed or frightened to ask adults to stop smoking which is why the regulations are an important step in protecting children from the harms of secondhand smoke.”