Proposals to ban smoking in cars with children present have hit an obstacle after Deputy Prime Minister Nick Clegg made clear he does not back legislation.
A Labour plan to give the Health Secretary powers to make smoking with children in the car an offence was approved by the House of Lords after 28 Liberal Democrat peers rebelled to inflict defeat on the Government.
The move could become law as Government sources indicated that MPs will be given a free vote on the issue when in the Children and Families Bill returns to the Commons.
But Mr Clegg made clear he will not back the scheme, saying that a ban would not be enforceable in practice.
While he said that parents who smoke while their children are in the car were "stupid", the Lib Dem leader said that it was wrong to try to "sub-contract" responsible parenting to the state.
He suggested it would be comparable to trying to legislate to stop children watching too much TV or drinking too many fizzy drinks.
Speaking on LBC 97.3 radio's Call Clegg show, the Deputy Prime Minister said: "I don't personally think that it is going to work to pass a law.
"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'm like anybody else, I've got small children, I'm dismayed that anyone might do that."
But he added: "The question is 'Is it right always to have a law to fix things you don't like?'. I know the temptation is to say 'There's a problem, where's the law?', but I am quite an old-fashioned liberal and 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.
"A family sailing along the M4 and mum and dad are smoking - how on Earth are you going to properly enforce that? I just think it's basic common sense - don't do it."
Mr Clegg added: "Laws and legislation are not always the solution. We have to live in a society where we expect adults to behave like adults and we expect parents to behave like responsible parents. You can't sub-contract responsible parenting to the state.
"It is deeply irresponsible to smoke in an enclosed space with your kids - don't do it."
Asked whether David Cameron backed a ban on smoking in cars with children present, the Prime Minister's official spokesman told a Westminster media briefing: "The Prime Minister wants to listen to the arguments.
"There was a debate in the Lords yesterday and it will be further discussed in the Commons in the coming period. There will be an opportunity for the Prime Minister and other parliamentarians to consider the issue and listen to the arguments."
Asked whether Mr Cameron thought a ban could be enforced, the spokesman said: "He wants to consider the arguments on all sides. I'm sure that issues around enforceability will be discussed and debated in the Commons and the issue will come to a vote."
The Labour amendment was backed by 222 votes to 197, majority 25, in the House of Lords, thanks to support from Lib Dem rebels including former Cabinet ministers Baroness Williams of Crosby and Lord Rodgers of Quarry Bank.
Shadow public health minister Luciana Berger said after the vote: "The Government should accept that tonight's defeat sends a clear message - we need action to put children's health first.
"When it comes to the Commons next month, ministers, as well as Tory and Liberal Democrat MPs, should show they recognise that this issue cannot be ignored and not try to kick it into the long grass."
British Lung Foundation chief executive Dr Penny Woods said: "We are absolutely delighted that the House of Lords has voted in favour of this move towards banning smoking in cars when children are present.
"If enacted, a ban would help protect the nearly half a million children who are exposed to toxic second-hand smoke in a car every week.
"This is an issue that the British Lung Foundation has been campaigning on relentlessly. We now look to the Government to listen to the overwhelming support shown across the House of Lords for a ban and take the necessary steps to protect our children's health."
Mr Clegg admitted that he has still not managed to stop smoking himself, telling LBC: "I am on an ongoing journey of weaning myself off. It's patchy progress.
"I hope my children aren't listening because I don't think they know I have ever touched a cigarette."