Police are still waiting to be given the power to issue fines to people smoking in cars with children a year after the crackdown was launched, according to the organisation that represents rank-and-file officers.
The Police Federation of England and Wales argued that a "piece of the jigsaw is missing" as officers cannot yet issue fixed penalty notices.
However, health campaign groups said the legislation has worked well and compliance has been high.
It became illegal in England and Wales to smoke in a car or other vehicle with anyone under the age of 18 present from October 1 last year, with motorists or passengers facing a £50 penalty.
But figures obtained through Freedom of Information requests have suggested enforcement of the ban has been minimal.
The BBC reported in June that only three forces reported incidents in the first seven months, and all were dealt with by verbal warnings.
Jayne Willetts, lead on roads policing for the Federation, said: "It's been really difficult for our members to enforce this law, because since the change of legislation police have still yet to be given the power to issue a fixed penalty notice (FPN).
"The original plan was for the public health authority to seek to change the law and give police extended powers.
"This would allow officers to stop motorists and issue on-the-spot fines, like they currently do for other offences, such as using a mobile phone while driving. But because that hasn't happened, a piece of the jigsaw is missing."
Society needs to take responsibility for the issue, Ms Willetts added.
She said: "At the end of the day, this is a societal issue, not just one that can be solved by law enforcement alone."
Action on Smoking and Health said a survey shows that 86% of children in 2016 have reported no exposure to smoking in cars compared with 83% last year.
According to the poll, the law prohibiting smoking in cars with under-18s is supported by 87% of adults in England - and it is also backed by three quarters (76%) of smokers.
ASH chief executive Deborah Arnott said: "Compliance with the legislation on smoking in cars with children is dependent on the level of public support, not enforcement action.
"And people are complying with this popular law which protects children from the harm caused by second-hand smoke."
She added: "It is not just children, adults are also at risk from exposure to the high concentrations of tobacco smoke found in confined spaces like cars.
"Prohibiting smoking completely would make the law simpler to enforce as well as protecting all car occupants from the serious harms caused by tobacco smoke."
A report produced by the Chartered Institute of Environmental Health (CIEH) and Improving Performance in Practice (IPIP) looking at compliance levels with the legislation found that more than eight out of 10 people knew there was such a law and all those interviewed agreed that such legislation should exist.
Ian Gray, principal policy officer for the CIEH, said: "Our study shows that compliance with the smoke-free private vehicles legislation has been high.
"People are aware of the law and during the surveys, at no point were children found in a car while an adult was smoking.
"In-fact, there is positive evidence of local authorities and police forces working collaboratively to enforce the new law.
"The legislation was introduced to protect children from the harmful effects of secondhand smoke and considering high compliance levels, this is another positive step for public health initiatives in England."