Later this evening, MPs will be given a vote on proposals to ban smoking in cars carrying children. It will be a free vote, and within each party there will be MPs who vote each way. If the proposal is approved, from next year it could become a criminal offence to smoke when there is a child in the car.

There would be no new police resource allocated to enforcing the ban proactively. But this would send a clear message out that smoking in cars with children is unacceptable, and I support the measure wholeheartedly.

Votes like this always raise questions about the role of the state in telling us how to live our lives. Many liberals are instinctively uncomfortable about government telling parents how to raise their children. I sympathise with those concerns.

But let us look at the facts. The Department of Health carried out a survey recently that showed 300,000 children every year visit their doctor because of illness linked to second-hand smoking. The toxic effects of smoking in a car are 11 times worse than in an open space. And, crucially, in a car there is no way that a child can escape the smoke even if they understand the dangers posed by smoke.

John Stuart Mill argued that “the only purpose for which power can be rightfully exercised over any member of a civilized community… is to prevent harm to others”. This principle forms the basis of the rule of law in a modern liberal society.

For decades, society has accepted the compelling scientific evidence that cigarette smoke does serious, lasting harm to those who inhale it. Passive smoking causes 165,000 new episodes of disease among children each year, including asthma, respiratory and ear infections, bronchitis, and reduced lung function. Parliament legislated to stop adults physically harming children by hitting them: it is absolutely consistent to legislate against subjecting children to the harm of smoking in cars.

Several people I have spoken to over the past few weeks have talked about a “slippery slope” – and have suggested that this legislation could lead on to other more intrusive proposals. I completely agree that as Liberal Democrats we must guard against unjustified intrusion in people’s lives. But ultimately MPs have a responsibility to consider each decision like this on its own merits, and in this case, the evidence is very clear. In my view the benefits to the 165,000 children who experience smoke-related health problems each year far outweigh the very modest restriction proposed to individual liberty.

If we want to give every child a fair start in life, we cannot ignore the health factors that impact on a child’s wellbeing and development. Protecting children from smoke in cars is an important step forwards in delivering a fairer society. And let us be very clear that responsible parenting requires more than complying with the law. Parents should try to protect their children from harmful exposure to smoke whenever they can – not just when they are in the car.

In years to come we will look back and ask ourselves – “did we really allow smoking in cars when a child was on board, at direct risk of harm?” Just like seatbelts, this is a very obvious and legitimate protection for children. I hope Parliament votes in favour tonight.

* Norman Lamb is MP for North Norfolk and was Liberal Democrat Minister of State at the Department of Health until May 2015. He now chairs the Science and Technology Select Committee