The Lords has backed a Labour plan to ban smoking in cars carrying children, despite opposition from the government.
Labour peers tabled an amendment to the Children and Families Bill detailing their proposal for England, which they said was about "protecting children".
Ministers had argued that the new law was a "blunt instrument" and public information campaigns were preferable.
But the BBC understands that government backbenchers will not be pressed to reverse the change in the Commons.
Instead, they will have a free vote on the amendment when the bill returns from the Lords.
A Downing Street spokesman said earlier that Prime Minister David Cameron was ready to "listen to the arguments".
The amendment empowers, but does not compel, the government at a later date to make it a criminal offence for drivers to fail to prevent smoking in their vehicle when children are present.
Labour has said that if the measure does not become law before the next election, it will be included in its manifesto.
Smoking was banned in England in workplaces and most enclosed public spaces in July 2007 following similar legislation in Scotland, Wales and Northern Ireland. The law prohibited smoking in vehicles used for work.
The amendment to the Children and Families Bill was brought forward by Lord Hunt of Kings Heath, Lord Faulkner and Baroness Hughes.
Conservative peer Lord Cormack argued that any law which "brings the state into the private space of individuals is to be deplored".
But Lord Hunt said: "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."
The level increased to 11 times when the car was not moving with the windows closed, he said.
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."
Shadow health secretary Andy Burnham told Sky News the move had "strong public support".
Campaigners say the developing lungs of children are much more vulnerable to the effects of second-hand smoke - which can be concentrated in cars - increasing their risk of illnesses that range from asthma and colds to lung cancer.
They have been calling for action for some time. In 2009, Prof Terence Stephenson, then president of the Royal College of Paediatrics and Child Health called for a ban in a BBC News website opinion column.
But the pro-smoking group Forest disputes such claims.
Director Simon Clark said: "Legislation is completely unnecessary. Most adult smokers accept that smoking in a car with children present is inconsiderate and the overwhelming majority choose not to.
"Education, not legislation, is the way forward."
Forest also argues that banning smoking in private vehicles would be almost impossible to enforce and a serious invasion of people's private space.
Health Minster Earl Howe said: "We all want to eradicate smoking in cars carrying children.
"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 also argued there were "substantial challenges" with enforcing a ban on smoking in cars particularly vehicles "travelling at speed", questioning whether people would comply with the law if they knew there was little chance of it being enforced.
But at the end of the debate peers backed the Labour amendment by 222 votes to 197.
Calls to prohibit smoking in private vehicles when children are present have been raised in Parliament on several occasions since the 2007 ban came into effect.
In 2011, proposals from Labour MP Alex Cunningham cleared their first legislative hurdle, before facing significant opposition from MPs of all parties.
The following year, Lord Ribeiro introduced a private member's bill to make offenders liable for a £60 fine or attendance at a smoke awareness course. It won approval in the House of Lords, although supporters admitted they did not have government backing for the move.
The Labour amendment backed by the Lords was initially proposed by Croydon North MP Steve Reed last April.
He won the support of organisations including the British Heart Foundation, Asthma UK, the Royal College of Paediatrics, and Child Health.
Although children's minister Edward Timpson said at the time that a ban "would not be easy to enforce", the government was researching the issue, and Mr Reed withdrew his amendment.
The Welsh government has said it would consider a ban should an awareness campaign not lead to a drop in children's exposure to second-hand smoke.
Meanwhile, in Scotland, Lib Dem MSP Jim Hume has indicated he will be presenting a bill this year to bring in a ban, while Northern Ireland's health minister has announced plans for a consultation on the issue.