Smoking in cars in the presence of children could be outlawed after the House of Lords voted for a ban.
MPs will now be granted a free vote on the plan, set out in in a Labour amendment to the Children and Families Bill, which was backed by the Lords by 222 votes to 197.
It comes as a defeat for the Government, which had argued that education campaigns were effective in discouraging smoking around children.
But Labour's Shadow Health Secretary Andy Burnham said that the Government was “duty bound” to use any measure that might improve children's health.
The Government's decision to grant a free vote on the issue makes it likely that the ban will come into force, with the majority of Labour MPs likely to be supported by a number Coalition members. The ban would only affect England.
Health campaigners said that the ban could protect nearly half a million children who are exposed second-hand smoke in a car every week.
Dr Penny Woods, chief executive of the British Lung Foundation, said that the Government should “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.”
The Lords also backed the introduction of standardised packaging for tobacco products in the UK. A review of the policy, by the leading paediatrician Sir Cyril Chantler is due to report in March, with the Government's final decision likely to be led by his findings.
A Department of Health spokesperson said: “Evaluation of [public health] campaigns shows they are increasing awareness of the risks of second-hand smoke as well changing attitudes and behaviours. We will consider the views expressed today alongside the evidence and respond in due course.”