Around 700 medics and health experts are calling on the Government to ban smoking in cars carrying children ahead of a Commons vote on Monday.
In a letter to the British Medical Journal (BMJ), respiratory experts said secondhand smoke was a "major cause of ill health in children", damaging the developing lungs, causing sudden infant death and leading to thousands of hospital trips every year.
Signatories to the letter are being co-ordinated by Dr Nicholas Hopkinson from Imperial College London and chairman of the British Thoracic Society's chronic obstructive pulmonary disease specialist advisory group.
As of 8am on Thursday, 692 doctors, nurses and other health professionals and experts had signed up to support the ban on smoking in cars.
Their letter said: "Secondhand exposure to tobacco smoke is a major cause of ill health in children.
"Smoke inhalation damages the developing lung, and the Royal College of Physicians estimates that each year in the UK it is responsible for 300,000 primary care contacts, 9,500 hospital admissions, at least 200 cases of bacterial meningitis and 40 sudden infant deaths.
"Most of this additional burden of disease falls on the more disadvantaged children in society, and all of it is avoidable."
The signatories said those objecting to a change in law assumed there was a " right to force children to breathe tobacco smoke". O bjectors "seem to value this more highly than the children's right to breathe clean air", they added.
They went on: "We urge MPs to support this important public health measure, which will protect the wellbeing of children now and in the future."
The letter comes as a minister announced yesterday there would be a Commons vote on the issue on Monday.
Transport minister Robert Goodwill told MPs he would vote in favour of the ban, having been forced to sit in the back of a car as a child while his father smoked.
The vote comes as the Children and Families Bill returns to the Commons.
The Bill was amended in the House of Lords last week after Labour tabled an amendment which would give the Health Secretary the power to make it illegal to smoke in a car carrying children. Peers have now accepted a Government-backed version of the amendment.
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. Government should take encouragement from that and focus on education not legislation.
"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."
Deborah Arnott, chief executive of health charity Ash, said: " Every day doctors see children with illnesses resulting from exposure to second-hand smoke, causing harm which is entirely preventable.
"We understand why they feel so strongly and support their call to put an end to children's exposure to smoking in cars."
Dr Hopkinson 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 secondhand 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 secondhand 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, chair of the Association of Respiratory Nurse Specialists, said: " Many people don't realise quite how serious secondhand 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.
"Education campaigns can help raise awareness of these dangers, but the evidence suggests they are a lot more effective in bringing about real behaviour change when accompanied by legislation."
Dr Penny Woods, chief executive of the British Lung Foundation, said: " 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."