Smoking in cars carrying children could be banned after peers defeated the Government on the issue.
The Labour move was backed by 222 votes to 197, majority 25, in the House of Lords.
Shadow health minister Lord Hunt of Kings Heath said it was a matter of "child protection".
He said: "Large numbers of children remain exposed to high concentrations of second-hand smoke when confined in family cars."
The successful amendment to the Children and Families Bill does not directly ban smoking in cars with children in them.
But it backs the principle of the ban by giving the Health Secretary a power to bring in regulations that would make it an offence for a driver to "fail to prevent smoking in the vehicle when a child or children are present".
Lord Hunt said members of the public were protected by the ban on smoking in work vehicles.
But he said: "Around one child in five reports being regularly exposed to second-hand smoke in cars with catastrophic health consequences."
The British Lung Foundation reported around 185,000 children aged between 11 and 15 are exposed to "potentially toxic concentrations" of second-hand smoke in their family car every day or on most days, Lord Hunt said.
He added: "I was very surprised by research that has been identified by the British Lung Foundation, which shows that a single cigarette smoked in a moving car with a window half open exposes a child in the centre of a backseat to around two-thirds as much second-hand smoke as in an average smoke-filled pub of days gone by."
He said the level increased to 11 times when the car was not moving with the windows closed.
Lord Hunt went on: "Some Lords will argue a car is a private space and that we should not legislate for what happens within such a space. But there are more important principles than that.
"For one for me is the need for child protection. Unlike most adults, children lack the freedom to decide when and how to travel, they lack the authority most adults have to ask people not to smoke in their company.
"And in those circumstances I think it is right for Parliament to step in to protect children."
Health Minister Earl Howe said the Government wanted to first try to encourage "lasting and positive behaviour change" but would not rule out future legislation.
He said the Government would launch a new campaign on smoke-free houses and cars this year.
He said: "We all want to eradicate smoking in cars carrying children. None of us want to see children continuing to be exposed to harmful second-hand smoke, whether that is in the home or the family car.
"The Government believes that encouraging lasting and positive behaviour change by making smokers aware of the significant health risks of second-hand smoke will be more effective than resorting to legislation, which is a blunt instrument to tackle the problem.
"I believe we should only consider resorting to using legislation if our work to promote positive changes in behaviour is shown not to have the required effect."
He said there were "substantial challenges" with enforcing a ban on smoking in cars particularly vehicles "travelling at speed".
"Currently local authorities enforce smoke-free legislation but they don't have the power or the means to require moving vehicles to move over," he said.
"We would therefore need to set up a complex and probably resource-intensive enforcement regime probably involving the police."
And he questioned whether people would "comply with the law" if they knew there was little chance of enforcement.
He added: "If we cannot credibly enforce the law, the law itself lacks credibility."
Lord Howe introduced amendments that would allow the Government to bring in legislation on plain packaging for tobacco following a review by Sir Cyril Chantler.
He also confirmed that the Government would introduce measures at the Bill's next stage to outlaw selling e-cigarettes to children and make it an offence to buy cigarettes to give to children.
Baroness Finlay of Llandaff, a former president of the Royal Society of Medicine, said a British Lung Foundation study showed 31% of children had asked someone to stop smoking in a car but 34% had been too frightened or embarrassed to ask.
Backing Lord Hunt's amendment, the crossbench peer said: "A child in the backseat belted in is effectively imprisoned in the vehicle for their own safety while travelling, but they really are stuck there.
"Whatever adults do they have no control over. But it is also worth remembering that they don't feel able to say anything."
Broadcaster Floella Benjamin said more than 800 children a day went to see doctors with problems associated with second-hand smoke.
Baroness Benjamin, a Liberal Democrat peer, said: "Let's take careful consideration as to what action we should take to protect from the result of second-hand smoking and act robustly in the best interests of the child."
Lib Dem Baroness Tyler of Enfield told peers: "Whenever a proposal of this sort is put forward, some will argue it is extending the reach of the nanny state - this is private space where the Government shouldn't get involved.
"But there are, I think, quite clearly overriding reasons for acting now to safeguard the most precious commodity of all, the health of our children.
"We hear lots of people, generally from the tobacco industry talking about the rights of smokers, but who is sticking up for the rights of the child?
"Surely every child should have the right to be in a safe environment."
Lord Stoddart of Swindon, arguing against the amendment, said cars were already full of petrol fumes.
The Independent Labour peer said: "It is going to be very difficult to enforce this sort of legislation. We are not yet able to enforce properly the use of mobile phones in moving vehicles.
"Anyone who travels along the motorways knows perfectly well that practically every lorry driver is using a mobile phone, and yet we know that very few of them are prosecuted let alone fined for doing so."
And he asked: "What is going to be the next step? The people who are against smoking will not stop at smoking in cars?
"They will then want to stop smoking in homes where there are children.
"That really would be an amazing interference in people's lives."
Lord Stoddart, 88, said he was born in the a small house in the Rhondda Valley and lived in a small house in which his grandparents and parents all smoked.
"I have to say it hasn't done me very much harm," he added.
Division lists showed there were 28 Liberal Democrat rebels on the amendment, including former Cabinet ministers Baroness Williams of Crosby and Lord Rodgers of Quarry Bank.
The amendment was also supported by 148 Labour peers, 34 crossbenchers, 11 others and the Bishop of St Albans.
The opponents of the amendment were 136 Tories, 26 crossbenchers, Labour peers Baroness Golding and Lord Howie of Troon, 29 Liberal Democrats and four others.