More than 700 doctors and other health experts have put their names to a letter urging MPs to back a ban in England on smoking in cars with children present.
The issue is due to be voted on in Parliament on Monday.
The signatories to the letter in the British Medical Journal say the move is needed "to protect the well-being of children now and in the future".
They include nurses, doctors and surgeons working across the NHS.
The ban being debated would apply to under-18s - as 18 is the legal age at which people can buy cigarettes.
The letter argues that second-hand smoke exposure is a "major cause of ill-health in children", particularly among the most disadvantaged groups.
It says smoking in cars exposes children to particularly "high amounts of tobacco smoke" and there is now a consensus that children should be protected from such unnecessary hazards.
It also says there are precedents to a ban, including laws to require people to wear seatbelts and, more recently, the ban on mobile phones while driving.
The signatories have been co-ordinated by Dr Nicholas Hopkinson, from Imperial College London, who is chairman of the British Thoracic Society's chronic obstructive pulmonary disease specialist advisory group.
He said: "This letter issues a powerful statement from the medical professionals of this country - the people who, every day, are treating illnesses brought on by second-hand smoke in children - about the rights of children to breathe clean air that won't make them sick.
"Next week, MPs have a chance to help protect children from the proven dangers of second-hand smoke.
"If they vote in favour, it could help protect the health of literally hundreds of thousands of children nationwide. If they vote against, it will go down in history as a huge missed opportunity."
Nurse Rebecca Sherrington, chairwoman of the Association of Respiratory Nurse Specialists, said: "Many people don't realise quite how serious second-hand smoke can be for children, especially in the concentrations that can build up in the car.
"Parents are often surprised that it can lead to illnesses such as ear infections, meningitis and cot death."
Dr Penny Woods, chief executive of the British Lung Foundation, added: "Bans on smoking in cars carrying children already exist and are being enforced in countries such as Australia, Canada, South Africa and the USA. It's about time that we started protecting children in this country too."
Tobacco specialist Peter Mackereth, from Manchester's Christie Hospital, said children's smaller undeveloped lungs and narrower airways struggled to cope with the sidestream of heavily concentrated cigarette smoke, which can contain carbon monoxide, arsenic and formaldehyde.
Rolling down car windows only served to funnel the smoke right back into the car towards the children in the backseats, he told BBC's Breakfast.
Simon Clark, director of smokers' lobby group Forest, said: "Smoking in cars with children is inconsiderate but there is a line the state shouldn't cross when it comes to dictating how people behave in private places.
"Very few adults smoke in a car with children these days. We urge MPs to reject this unnecessary intrusion into people's private lives and trust parents to make the right decision for their children without the need for heavy-handed state intervention."
The vote by MPs comes after the House of Lords last week backed a Labour amendment to the Children and Families Bill.
The amendment empowers, but does not compel, the government to make it a criminal offence for drivers to fail to prevent smoking in their vehicles when children are present.
The government has now told its MPs they can have a free vote on the issue.
Labour has said that if the measure does not become law before the next election, it will be included in its manifesto.
Calls to prohibit smoking in private vehicles when children are present have been raised in Parliament on several occasions since the 2007 ban on smoking in public places came into effect.
The Welsh government has said it would consider a ban should an awareness campaign not lead to a drop in children's exposure to second-hand smoke.
Meanwhile, in Scotland, Lib Dem MSP Jim Hume has indicated he will be presenting a bill this year to bring in a ban, while Northern Ireland's health minister has announced plans for a consultation on the issue.