Boris Johnson has joined l eading medics in urging MPs to vote in favour of outlawing smoking in cars carrying children.
The Mayor of London said the practice was so "disgusting" and harmful to youngsters' health that even libertarians such as him should welcome it.
The Commons is expected to approve giving the health secretary the power to impose a ban despite the opposition of some MPs including members of the Cabinet.
Ministers have been granted a free vote on the measure - successfully introduced by Labour in a House of Lords amendment to the Children and Families Bill - meaning they are not tied to a party line.
Prime Minister David Cameron, who has declined to be drawn on his personal view, is expected to miss the vote to concentrate on the Government's response to flooding crisis.
Health Secretary Jeremy Hunt is among his colleagues backing the move while Justice Secretary Chris Grayling is in the "no" camp of those who say it is unenforceable.
Deputy Prime Minister Nick Clegg has spoken out against attempts to "sub-contract responsible parenting to the state" and pro-smoking groups have branded it an "unnecessary intrusion".
Last week hundreds of medics and health experts signed a letter in support of the ban which is backed by cancer charities and the Royal College of Physicians (RCP).
Mr Johnson used his column in the Daily Telegraph to appeal to party colleagues to accept that the "bossyboots brigade" he so often rails against were right in this case.
"Surely to goodness - you might say - people these days are aware of the problem of passive smoking? Surely all smokers know that they shouldn't be puffing away in a car, while the pink defenceless lungs of kids are sucking in the evil vapours?" he wrote.
"Alas, I am afraid that people either don't know, or don't care enough."
"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."
John Britton, chair of the RCP's t obacco advisory group, said that e very year 160,000 children are adversely affected by second-hand smoking, costing the NHS in England more than £23 million.
" The evidence shows the importance of protecting children from passive smoking, and cars are still a major area of exposure," he said.
"Such exposure is widely associated with adverse health effects, including an increased risk of respiratory and allergic symptoms.
"That is why the RCP is calling on MPs to support this important public health amendment."
Transport Minister Robert Goodwill told MPs last week 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.
Fellow Conservative Jacob Rees-Mogg said he feared it was an authoritarian step that would quickly lead to calls for people to be banned from smoking around children in their own homes.
"Once it is admitted that the State can intrude on people's legal activities, then it is hard to establish the point at which it must stop," he wrote in the Mail on Sunday.
"I am very fortunate in having a marvellous nanny, Miss Veronica Cook, who after 47 years is still looking after my family.
"Nothing is more reliable than nanny, nor more objectionable than the nanny state."
Dr Penny Woods, British Lung Foundation chief executive, said a ban on smoking in cars carrying children is successfully enforced in several countries, including Canada and Australia.
She said: "MPs have a real chance of making history by voting to give children in the UK the same protection."
A poll by Cancer Research UK revealed that almost a quarter of people underestimate the amount of deaths which are associated with smoking.
The charity found that 23% of people believe that 10,000 people or fewer die as result of tobacco-related diseases in the UK every year, but the death toll associated with smoking is actually around 100,000, a spokesman said.