Lib Dem leader Nick Clegg has suggested a proposal to ban smoking in cars carrying children would be comparable to trying to legislate to stop a child watching too much TV or drinking too many fizzy drinks.
The Deputy Prime Minister made it clear he would not back such a ban after the Labour plan was approved last night by the House of Lords.
Speaking on LBC radio, Mr Clegg said: "Of course it is a stupid thing to do to smoke in a car with kids in the back, of course it is - in the same way you shouldn't give your children a can of Coke before going to bed or only feed them on crisps breakfast, lunch and supper.
"I think you shouldn't legislate unless you think it is going to make a difference, and I don't see how this is going to be enforced. "
Downing Street said David Cameron was ready to "listen to the arguments" about a ban on smoking in cars that are carrying children.
The Prime Minister's official spokesman told a media briefing, "This is an issue that is going to be discussed and debated in Parliament today, and the Prime Minister's view is that he wants to listen to the arguments."
Pro-smoking group Forest has accused Labour of "playing politics" with its attempt to ban smoking in cars carrying children and said the measure would be difficult to enforce.
Director Simon Clark said: "We think legislation is a very heavy-handed way to tackle this problem.
"I don't think it is as big a problem as they like to make out. The vast majority of smokers wouldn't dream of lighting a cigarette in a car with a child.
"If you say 'Let's ban smoking in cars with children present', are you going to go on and try and ban smoking in the home if children are present? I think this would set a very bad precedent and lead to a lot of problems."
Shadow public health minister Luciana Berger said legislation and education "can work very well hand in hand" to ensure drivers do not smoke when there are children in the vehicle.
When asked if launching an educational campaign would be more effective than enforcing a ban Ms Berger told Daybreak, "I don't think it's either or."
"I think we need to look at education as a lot of people don't know that smoke within a car is 23 times more toxic than it is smoking in a building or a home, and that's obviously very important that people know that", she continued.
"But we also know that legislation alongside an education campaign can make a massive difference."
Damage done to children's lungs by second hand smoke "may not be reversible", a health expert warned.
Professor Stephen Spiro, Deputy Chairman of the British Lung Foundation, told Daybreak children's lungs are still growing into their mid-teens, so any damage done by cigarette smoke may well be permanent.
"The problem really is that smoking in cars is extremely bad for children - 300,000 visits to GPs are generated by bad soar throats, ear ache, worsening of asthma and other childhood conditions.
"The lungs in children grow until they are in their mid-teens, so that damage may not be reversible."