A new law comes into force in Scotland on Monday banning smoking in cars when children are present, as part of the government's plans for a "tobacco-free generation".

Under the law smokers face a £100 ($127, 119 euros) on-the-spot fine if caught lighting up in a private vehicle where under-18s are present.

The penalty increases up to £1,000 if a case goes to court, under a bill passed unanimously in Scottish Parliament in December.

The new measure was introduced as part of the Scottish government's plans to lower the smoking rate to under five percent by 2034.

"It's simply not safe to smoke when a child is in the car. Dangerous levels of chemicals can build up, even on short journeys," said Aileen Campbell, Scotland's public health minister.

Breathing second hand smoke is linked to asthma, respiratory infections, lung cancer and coronary heart disease, according to the World Health Organization.

The law has been welcomed by health charities, with Ash Scotland saying it sends a clear message that children should grow up in a smoke-free environment.

"We know from speaking to parents that they want to protect their children from tobacco smoke, but often don't know enough about how smoke is harmful and lingers in the air even after you can't see or smell it," said the organisation's Chief Executive Sheila Duffy.

But the move has been criticised by smokers' group Forest, whose director Simon Clark described the law as "patronising and unnecessary".

"Very few adults smoke in cars with children. Smokers know it's inconsiderate and the overwhelming majority don't do it.

"So few people smoke when there's a child in the car it will be like looking for a needle in a haystack," he said.

The law banning smoking in vehicles carrying children came into force in England and Wales in October 2015.