It is now illegal to smoke in a vehicle carrying children in Northern Ireland.
Smoking in 'enclosed' cars in the presence of anyone under the age of 18 is against the law from Monday, February 1.
Anyone who breaks the law faces fixed penalty notices of £50, and maximum fines of up to £2,500 upon conviction.
The Northern Ireland Executive's new rules note that a vehicle is counted as 'enclosed' even if windows or doors are open.
A leading lung charity has welcomed the move, saying: "It's a huge win for children’s lung health."
Legislation had already been in place which bans smoking in certain premises, places and vehicles including on public transport and in work vehicles used by more than one person.
The new regulations extend the current smoke-free provisions to private vehicles where children are present, when there is more than one person in the vehicle, and the vehicle is enclosed.
It's not only an offence to smoke in a private vehicle where children are present - failing to prevent someone else smoking in that circumstance is also against the law.
Enforcement authorities will have the option of issuing fixed penalty notices of £50 to the driver of the vehicle if they break the law.
A conviction over the offence can lead to fines of up to £50 for the person smoking, which is reduced to £30 if the fixed penalty notice (FPN) is paid within 15 days.
The maximum fine is £1,000 if the smoker prosecuted and convicted by a court.
Drivers permitting others to smoke in a private vehicle will receive a £50 FPN, or a maximum fine of £2,500 if prosecuted and convicted by a court.
Exemptions have been made for caravans and mobile homes, if their primary use is as accommodation. However, the rules do apply when they are on the road.
Laying out the plans last year, Health Minister Robin Swann said Ireland the intent was to protect people, particularly children, from the effects of second-hand smoke, and to reduce smoking prevalence more broadly.
The NI Executive said a survey of young people aged 11-19 found around one in five reported the adult smokers they lived with also smoking in vehicles.
Research showed children and young people are more vulnerable to the harmful effects of second hand smoke as they breathe more rapidly than adults, and inhale more pollutants per pound of body weight.
The new law will be enforced by both local councils and the PSNI.
Northern Ireland is the last region in the UK to make the move, following England and Wales implementing the ban in 2015, and Scotland and the Republic of Ireland in 2016.
New vaping legislation to ban the sale of e-cigarettes to under 18s will also come into effect from February 1, mirroring current offences relating to tobacco sales.
Naomi Thompson, Health Improvement Manager at Cancer Focus NI said: “This is a significant move to a tobacco-free Northern Ireland.
"We firmly believe that these new laws will protect children’s health. We have already protected adults in workplaces and public places from second-hand smoke and it’s high time we gave our young people the same protection.
"It’ll also reduce their perception that smoking is normal behaviour. We know that children who regularly see adults smoke are more likely to try smoking.”