Smoking in your car should be against the law, doctors have told the Government.
The British Medical Association (BMA) is calling on ministers to bring in the "bold and courageous" ban for reasons of health rather than road safety.
Evidence suggests smoking in a closed vehicle exposes the occupants to large amounts of harmful chemicals, the doctors reported. Toxin levels can be 23 times higher than in a typical smoky bar, it is claimed, with children and the elderly said to be at particular risk.
Children absorb more pollutants than adults and their immature immune systems are less able to cope with the effects of second-hand smoke, according to the BMA.
The elderly are prone to respiratory problems that can be made worse by inhaling cigarette smoke, the doctors warned. Such vulnerable groups may be unable to refuse journeys in smoky vehicles, the BMA points out.
Dr Vivienne Nathanson, the BMA's director of professional activities, said: "Every year in England there are over 80,000 deaths that are caused by smoking. This figure increases to a shocking six million worldwide.
"But behind the stark statistics, doctors see the individual cases of ill-health and premature death caused by smoking and second-hand smoke. For this reason, doctors are committed to reducing the harm caused by tobacco.
"The UK made a huge step forward in the fight against tobacco by banning smoking in all enclosed public places but more can still be done. We are calling on UK governments to take the bold and courageous step of banning smoking in private vehicles."
Prime Minister David Cameron has distanced himself from Mr Cunningham's proposal. Speaking during Prime Minister's Questions in the Commons, Mr Cameron - an ex-smoker - said he supported the current smoking ban in public places.
But he added: "I am much more nervous about going into what people do inside a vehicle."