Health campaigners welcomed the legislation making it illegal to smoke in vehicles in which under-18s are travelling when it was introduced in October last year.
It was hoped the law, under which drivers can be issued with a £50 fine, would help to protect children from the dangers of second-hand smoke, which has been linked to chest infections and asthma.
But a Freedom of Information request made by the BBC to police forces in England and Wales suggests that officers may be struggling to enforce the legislation.
The figures show that police forces have issued no fines to drivers under the new law, with officers choosing instead to give verbal warnings.
According to the data from 42 forces in England and Wales, two warnings were issued by the Metropolitan Police, four by Dyfed-Powys Police and one by Devon & Cornwall Police.
The findings have raised doubts over the effectiveness of the law, which often requires officers on the ground to make split-second decisions based on quick glances inside a car.
Nigel Rabbits, of Devon & Cornwall Police Federation, told the BBC: “It's very difficult to enforce because you are talking about looking at a vehicle and trying to figure out what's going on inside.
“If you're looking for someone under the age of 18 that's difficult without stopping the vehicle and once the vehicle has been stopped getting the evidence for prosecution is extremely difficult.”
But campaigners have reaffirmed their commitment to the law after the FOI findings, saying the legislation was intended to change attitudes rather than punish drivers.
Figures from the British Lung Foundation found that more than 430,000 children could be exposed to second-hand smoke in cars each week.
The law, which relates to England and Wales, was seen as a huge step forward in tackling this problem, applying to both drivers and passengers who smoke with under-18s, though not to those travelling alone or in convertibles with the roof down.
Scotland is to introduce a similar ban after MSPs voted in favour of a change to the law in December last year.
Responding to the FOI figures, the Department for Health said: “In changing the law we always said the measure of success would be in changes in attitude and behaviour, not number of enforcement actions.”