The MSP pushing for a ban on smoking in cars when children are present has said there is a "moral duty" to act, as he dismissed claims the legislation would be unenforceable.

Liberal Democrat Jim Hume wants a £100 fine to be brought in for people caught smoking in vehicles if anyone aged under 18 is inside.

He denied suggestions that his member's Bill seeking a change in the law is "flawed" as he faced questions from Holyrood's Health Committee.

The Smoking Prohibition (Children in Motor Vehicles) (Scotland) Bill has won the support of the Scottish Government, but Police Scotland has raised concerns that it could lead to officers being diverted away from work to tackle terrorism or organised crime.

Under Mr Hume's proposals, if a ban is brought in officers from Police Scotland would be charged with enforcing it.

He told the committee that 60,000 youngsters a week are exposed to second-hand tobacco fumes in vehicles, with children more at risk of developing health issues such as wheezing, asthma, respiratory tract infections and lung cancer.

"Most affected children have no other transport option or are too young to make other arrangements and are not empowered to change the behaviour of adults around them," he said.

"I believe we have a moral duty to protect those children from the immediate health impacts of second hand smoke, give them the best start in life and support them to go on to lead healthy lives themselves."

The Liberal Democrat was questioned by MSPs, including Conservative Nanette Milne and the SNP's Dennis Robertson, about how the legislation would be enforced.

He responded: "We know in the last year 2013/14 Police Scotland detected over 36,000 seatbelt offences and 34,000 mobile phone offences.

"That's 70,000 in all in one year. So detection is very similar to that, that's actually seeing someone smoking, it's pretty obvious when someone is smoking in car."

Mr Hume acknowledged it could be more difficult to determine the age of older children in cars, but was "not impossible".

Mr Robertson said he believes the legislation is "somewhat flawed", adding: "Is it not better to try and raise the education and awareness of the harms of exposing children to smoke rather than imposing legislation which would appear, at the moment anyway, to be unenforceable?"

Mr Hume said: "As we can see from other countries I would dispute that.

"It has been enforceable but what we have seen is that legislation has acted as a deterrent and behaviour has changed, which I think is what we're all wanting."

The SNP's Richard Lyle, a smoker, said: "Everyone is going to start to report people they see smoking in their cars. Do you think that's not a bit over the top?"

Mr Lyle also raised the suggestion that the Bill could be the "thin end of the wedge" in a move to target smokers in cars with no children present, or in the home.

Mr Hume responded: "It's very clear, this Bill is very tight, it is only to do with the protection of children. If people want to smoke wherever they want to smoke that's fine.

"This law cannot be amended to make it anything to do with what people do in their houses."