Leading medics have urged MPs to vote in favour of a ban on smoking in cars.
Every year 160,000 children are adversely affected by second-hand smoking, costing the NHS in England more than £23m, the Royal College of Physicians (RCP) said.
MPs will vote today on whether or not people should be prohibited from smoking in cars with youngsters present as the Children and Families Bill returns to the House of Commons.
John Britton, chair of the RCP’s tobacco advisory group, said: “The evidence shows the importance of protecting children from passive smoking, and cars are still a major area of exposure.”
Boris Johnson has joined the call. The Mayor of London said the practice was "disgusting" and harmful to youngsters who were defenceless and "could not protest".
In a column in the Daily Telegraph, Mr Johnson said: "These kids cannot protest, and very often the smoker in the vehicle lacks the will to stub it out. This law would give that smoker that extra legal imperative to obey their conscience and do the right thing."
Rejecting critics' claims that it would divert police resources from more serious crimes, he said it would be "largely enforced by the natural social pressure of disapproval backed by law".
"So I apologise to all my libertarian chums: I am afraid on this one I am firmly with the bossyboots brigade. Ban smoking with kids in the car. It is a disgusting thing to do, and endangers their health. The proposal before parliament is a good one that will save lives."