More than 430,000 children between the ages of 11 and 15 are exposed to second-hand smoke in their families' cars at least once a week, according to a charity.
The British Lung Foundation (BLF) extrapolated the figures from a poll of 7,500 secondary school children in England.
Of those questioned in the survey, six per cent said they were exposed to smoke every day or most days in their family vehicle, with an additional eight per cent noting that they were exposed to second-hand smoke in cars once or twice a week.
The publication of the estimates comes just days ahead of a debate in the House of Lords, which will discuss the possibility of banning smoking in a vehicle when a child is present.
Dr Noel Snell, director of the BLF, said: "These shocking new figures clearly highlight the extent to which children are still being exposed to second-hand smoke when travelling in a car. And given this data only covers children aged between 11 and 15, it is possible that the total number of children affected on a weekly basis could be in excess of half a million."
"This amendment is fundamental to child protection and must be passed in the Lords if we are to help shape a healthy future for this generation of children," he added.
However, the debate is set to rage on as Smokers' lobby group Forest believes that the data published by the BLF is misleading and a ban on smoking in vehicles carrying children would be "disproportionate" to the scale of the problem.
Simon Clark, director of Forest, said: "According to surveys, only a very small number of adults still smoke in cars with children present. It's inconsiderate and most adults recognise that. Legislation is disproportionate to the problem. It would be very difficult to enforce and would be a huge waste of police resources. Education has to be better than coercion."
Baroness Tyler, who is calling for a ban on smoking in cars with children, said that the case for a ban of the practice was "indisputable", as she pointed out adults are protected from the dangers of second-hand smoke in their workplaces, public spaces and in work vehicles.
"We know that many children are too frightened or uncomfortable to ask adults to stop smoking when they are in a car," she concluded.