Smoking in cars with children is a bad idea.
While the dangers of passive smoking are typically overstated, the evidence of harm to children from smoking regularly in enclosed spaces is strong. Children are particularly vulnerable to smoke because their lungs and immune systems are less well developed. Smoking in a car with your kid in the back seat is bad parenting, plain and simple. If this is something you do, you should stop doing it.
A ban would not be that difficult to enforce. Saying a new law is 'unworkable' is one of the great refrains of British politics. In fact, most laws are highly workable, not least because Brits tend to follow the rules. If we've banned smoking in public places we'll be able to ban it in cars too.
The problem with the policy, which Labour is pushing through via an amendment to the children and families bill in the Lords today, is that it allows the state into your private property to demand that you make healthy life choices and threaten you with legal sanctions if you don't.
This is the thin end of the wedge - but what a great location for it. The anti-smoking lobby has found the ideal argument to press its advantage.
It has the public onside. A YouGov poll from 2011 found 78% of adults believe smoking should be banned in cars carrying children younger than 18. Forty-four per cent want it banned in all cars. You can see why shadow health secretary Andy Burnham has promised to bring the ban into law if Labour get to power, should today's efforts fail.
And the moral argument also seems coherent. This is not to hinder the freedom of adults, the anti-smoking brigade says, it's to protect children from the health risks of their parents' choices.
But if we're prepared to pass laws to protect children from the health risks of their parents' choices, then we'd better be prepared for the repercussions.
Under the same moral thinking, it would be acceptable for the state to arrest pregnant women who eat junk food all the time. Or perhaps it could make it illegal for parents to allow their children to lead sedentary lifestyles without enough exercise. The health repercussions of these choices are also severe.
Both these options would be morally consistent with banning smoking in cars with children. One would give the state power over women's bodies and the other would trigger state enforcement of lifestyle choices.
This is as pure an example of state over-reach as can be imagined. This is the state coming onto your private property and telling you how to live.
It starts with the car, but should the amendment pass the next stage will be banning smoking in the home where there are children. And finally legislation will be demanded to prescribe the behaviour of pregnant women.
First the car, then the home, then the body itself. State interference only goes one way. Once you invite it into your home it will make itself comfortable and demand ever greater powers.
Smoking in cars with children is bad. It shouldn't happen. There should certainly be public information campaigns shaming those parents who do it into stopping.
But the question is not whether people should smoke in cars with their children. The question is what we sacrifice if we use the law to stop it. In this case the sacrifice is too big.
Ian Dunt is editor of Politics.co.uk.
The opinions in politics.co.uk's Comment and Analysis section are those of the author and are no reflection of the views of the website or its owners.