A ban on smoking in cars carrying children could become law in England if MPs follow peers and vote in favour.
The House of Lords on Wednesday backed a Labour amendment to the Children and Families Bill 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 government has now told its MPs they can have a free vote on the issue, expected within weeks.
That means there will not be any party orders to oppose the measure when the Lords amendments to the bill are debated and increases the chances of them being voted through.
Any vote to ban smoking in cars carrying children would only affect England as the issue is the responsibility of the devolved governments elsewhere in the UK.
The amendment empowers, but does not compel, the government 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.
Shadow public health minister Luciana Berger: “It is about protecting children.”
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 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 added: “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. Children lack the freedom to decide when and how to travel.”