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."