Ahead of a House of Commons vote, London Mayor Boris Johnson urged MPs to vote in favour of outlawing smoking in cars carrying children in England.
Johnson said that, despite his libertarian instincts, the practice was so "disgusting" and harmful to children's health that he would welcome government intervention.
MPs in the Commons have a free vote later Monday and are expected to approve giving the health secretary the power to impose a ban despite the opposition of some lawmakers including members of the Cabinet.
The proposal was introduced by Labour in a House of Lords amendment to the Children and Families Bill, but even if it is passed it would not immediately mean a change in the law.
Last week more than 700 doctors, nurses and other health professionals signed an open letter to the British Medical Journal (BMJ) urging MPs to support the measure.
Johnson used his column in the Daily Telegraph to appeal to party colleagues to accept that the "bossyboots brigade" he often criticises were right in this case.
"Surely to goodness -- you might say -- people these days are aware of the problem of passive smoking?" Johnson wrote.
"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?
"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."
Meanwhile, Deputy Prime Minister Nick Clegg -- himself a smoker -- has said he does not support a ban.
Clegg said it was "a stupid thing to do when a child is in the back of a car" but said he did not want to "sub-contract responsible parenting to the state".
Prime Minister David Cameron, who has declined to be drawn on his personal view on the issue, is expected to miss the vote to focus on handling flooding in southern England.