More than 800 children visit their doctor every day due to the effects of being exposed to second-hand smoke, according to research published by the Royal College of Physicians.
The figures have been highlighted as the Government launches a campaign to increase awareness of the hidden dangers of smoking in homes and cars. Millions of children in the UK are exposed to second-hand smoke daily, which puts them at increased risk of lung disease, meningitis and cot death.
A survey found that of 679 smoking parents, 68% of them who smoke admit to doing so in the car with their children present while 75% of smoking parents were shocked to hear that second-hand smoke affects the health of so many children.
More than 80% of second-hand smoke is invisible and odourless, and contains harmful cancer-causing toxins and poisons.
Television adverts will show that smoking out of a car window or the backdoor is not enough to protect children from second-hand smoke.
Chief Medical Officer Professor Dame Sally Davies said: "It's well-known that smoking kills but many smokers still don't realise the damage their smoke causes to those around them. Second-hand smoke can be an invisible killer and with more than 300,000 people seeing their GP each year because of it, we need to make sure people know how dangerous it can be."
Professor Kevin Fenton, director of health and wellbeing at Public Health England, said he hoped the figures would be a "wake-up call". He added: "We hope these figures showing the number of children who need hospital treatment for the effects of second-hand smoke and the information contained in the campaign will provide a wake-up call to many smokers out there."
Dr Harpal Kumar, chief executive of Cancer Research UK, said it is "vital" that children are protected from the dangers of second-hand smoke. He added: "Raising awareness of the dangers, providing information and supporting parents to make healthy choices are the first steps towards this."
Dr Penny Woods, chief executive of the British Lung Foundation, said secondhand smoke "affects an unacceptable number of children". She said: "We cannot afford to wait any longer to protect the health of vulnerable children."
Doireann Maddock, senior cardiac nurse at the British Heart Foundation, said: "The single most effective way of reducing children's exposure to secondhand smoke is for parents to quit but, if this isn't possible, smokefree homes and cars offer the best alternative to help protect children from the effects of passive smoking."