Under new laws going into effect, drivers in England and Wales who smoke in their cars with children present as passengers can be cited and face fines, officials say.
If children under 18 are in the car, smokers — either the driver or adult passengers — can be fined £50 ($75) for smoking, even if the car's windows are down or its sunroof is open.
However, the law will not apply to people smoking in a convertible with its top down.
Police officials say they will hold off issuing citations at first to give the public time to become aware of and accustomed to the new law.
Police would pursue an "educational, advisory and non-confrontational approach" during the first three months of the ban, the National Police Chiefs' Council said in a statement.
"This would see people being given warnings rather than being issued with fines," the council said.
Health advocates were quick to hail the measure.
"Today is truly a cause for celebration for all those who care about protecting the health of generations to come," said Penny Woods, chief of the British Lung Foundation.
More than 430,000 children riding in cars are exposed to second-hand smoke each week, the foundation says.
Anti-smoking campaigners have called the car smoking ban the most significant step forward since smoking in public places was banned in the UK in 2007.
Several U.S. states, including California, Utah and Maine, have passed similar bans on smoking in vehicles where children are passengers, with similar measures pending in a number of other states.
Some in the UK argue the ban is essentially unenforceable.
"If drivers are spotted smoking will they be stopped in case there's a child in the back? The authorities, especially the police, must have better things to do," said Simon Clark, head of the smokers' lobby group Forest.
Steve White, chairman of the Police Federation, agreed the law would be "extremely challenging" to enforce, especially in an era of shrinking police budgets.
"The public needs to decide what it is that they want their police service to do because we are at breaking point," he said. "Should we be focusing on people smoking in cars with children in the cars or should we be focusing on burglaries?"
Some health officials have acknowledged the introduction of the legal ban is mainly intended to modify people's attitudes about smoking and health.
"Ultimately, what we are looking for is that people change their behavior," says Dr. Gina Radford, the Deputy Chief Medical Officer for England. "We are hoping that the very fact that we have this law will make people think about smoking in front of children wherever they are."
The Scottish Parliament reportedly considered a similar ban.
