From today it will be an offence for a person of any age to smoke in a private vehicle that is carrying someone who is under the age of 18.
Anyone caught breaching the ban will be slapped with a £50 fixed penalty fine.
The new law doesn't apply to e-cigarettes.
Another change to come in force tomorrow will make it illegal for retailers to sell e-cigarettes or e-liquids to someone under 18 for adults to buy (or try to buy) tobacco products or e-cigarettes for someone under 18 to smoke in private vehicles that are carrying someone under 18.
The changes are aimed at reducing the risk of damage being caused to young people by cigarette smoke.
Every time a child breathes in secondhand smoke, they breathe in thousands of chemicals. This puts them at risk of serious conditions, such as meningitis, cancer and respiratory infections such as bronchitis and pneumonia. It can also make asthma worse.
Secondhand smoke is dangerous for anyone, but children are especially vulnerable, because they breathe more rapidly and have less developed airways, lungs and immune systems. Over 80 per cent of cigarette smoke is invisible and opening windows does not remove its harmful effect.
The law is changing to protect children and young people from such harm.
The legislation covers any private vehicle that is enclosed wholly or partly by a roof. A convertible car, or coupe, with the roof completely down and stowed is not enclosed and so isn’t covered by the legislation. But a vehicle with a sunroof open is still enclosed and so is covered by the legislation.
Sitting in the open doorway of an enclosed vehicle is covered by the legislation.
The rules apply to motorhomes, campervans and caravans when they are being used as a vehicle but don’t apply when they are being used as living accommodation.
The rules don’t apply to boats, ships and aircraft, as they have their own rules work vehicles and public transport, as they are already covered by smoke-free legislation.
Health campaigners have hailed the move as a ‘tremendous victory’ and the most significant milestone since the ban on smoking in public places was introduced in 2007.
Deborah Arnott, chief executive of health charity ASH, said: “Making it illegal to smoke in cars with children is supported by the vast majority of adults, including smokers, who recognise the harm caused by second-hand smoke.
“But despite the level of public support some adults still smoke in front of children. We hope the legislation will make them realise they should stop, not just in their cars but also in their homes.”
Smokers’ lobby group Forest said the law stigmatises millions of decent people.
Director Simon Clark said: “The new law is unnecessary and almost certainly unenforceable.
“The overwhelming majority of smokers know smoking in a car carrying children is inconsiderate and they don’t do it.
“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.”
More about changes to law on smoking in vehicles at gov.uk.