London Mayor Boris Johnson is joining leading medics in urging MPs to vote in favour of outlawing smoking in cars carrying children.
Johnson says the practice is so "disgusting" and harmful to youngsters' health that even libertarians such as him should welcome it.
The House of Commons is expected to approve giving the government health secretary the power to impose a ban despite opposition from some MPs including cabinet members.
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 the UK's floods.
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 tobacco advisory group, said that every year 160,000 children are adversely affected by second-hand smoking, costing the NHS in England more than STG23 million ($A42 million).
