Health campaigners have welcomed the launch of a consultation which brings a ban on smoking in cars carrying children a step closer.
Campaign group Action on Smoking and Health (Ash) said children who are passengers in cars where someone is smoking are exposed to dangerous levels of toxic fumes.
The charity urged ministers to bring in the regulation banning smoke in vehicles where children under the age of 18 are present, in England, before the next parliament.
The consultation on the initiative launched today and will run for six weeks.
"Second-hand smoke is a real and substantial threat to child health," the consultation document states.
"Today, many children in England report being exposed to second-hand smoke in private vehicles, including the family car.
"The aims of these proposed regulations on smoking in private vehicles carrying children would be to: protect children from the health harms associated with exposure to second-hand smoke in private vehicles; encourage action by smokers to protect children from second-hand smoke; and in time, lead to a reduction in health conditions in children caused by exposure to second-hand smoke.
"The purpose of this consultation is to seek views on draft regulations before they are made."
In February, MPs voted in favour of legislation that could see the move brought forward under the Children and Families Act.
"Cars are small tin boxes where concentrations of tobacco smoke can reach dangerous levels very quickly," said Deborah Arnott, chief executive of Ash.
"As David Cameron himself has said, the time has come for it to be illegal to make children breathe in these toxic fumes. Laws stopping smoking in cars with children are popular with the public, with parliament and with children and we urge the Government to bring them into force before the next election."
A similar consultation will be launched in Wales "shortly", a Welsh Government spokesman said.
Wales' First Minister Carwyn Jones said: "In 2012, we launched the Fresh Start Wales campaign to promote smoke-free cars carrying children and raise awareness to parents and others the risk their smoking poses to the health of children. We made clear throughout that campaign we would consider the possibility of legislation when evidence of prevalence of smoking in cars carrying children in Wales is available in the summer of 2014.
"A sizeable minority of young people are still being exposed and adults continue to smoke in their cars when children are present.
"There is also evidence from the primary school survey that inequalities in children's exposure to second-hand smoke remains, so we will press ahead with plans to ban people smoking in cars carrying children."
On the English consultation, Dr Penny Woods, chief executive of the British Lung Foundation, said: "We are now closer than ever to the introduction of a law that would help prevent hundreds of thousands of children from being exposed to second-hand smoke in a car.
"We therefore expect this consultation will confirm all the details before a ban is finally introduced next year.
"There may well be opposition from the tobacco industry and the campaigning groups they fund, but the evidence of a need for a ban, and its effectiveness, is overwhelming."
Simon Clark, director of smokers group Forest, said: "A ban is excessive and unnecessary. Smoking in cars with children has been in decline for years. Today very few people do it because the overwhelming majority of smokers accept that it's inconsiderate.
"According to research less than 10% of people will be affected by legislation, which will be impossible to enforce, so what's the point?
"Banning smoking in cars with children is a classic case of government flexing its muscles and introducing legislation just because it can."