If the government enforces proposals to ban adults from smoking in cars with children then it may as well "get out the fat calipers" whenever "lardy children" are seen on the street, a Conservative MP has said.
On Monday evening MPs overwhelmingly backed a measure that would allow the health secretary to outlaw smoking in vehicles carrying children.
Ministers were 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 were not tied to a party line.
Anne Main, the Conservative MP for St Albans, said she had never "heard of a more illiberal, nonsensical and unenforceable proposal" than the idea of banning smoking in cars.
"I cannot think that this proposal will be enforceable," she told MPs. "We all want to protect children. In that case, perhaps we should get out the fat calipers when we see very lardy children walking down our high streets because their parents feed them junk of an evening.
"Perhaps we should ban fattening foods because there are more than a million people with type 2 diabetes, as has been said in the media today.
"Where will it stop? We need to educate people. We need to ensure that parents do what is best for their children because they believe in doing what is best for them. We cannot legislate every single risk and danger out of existence."
Tory and Liberal Democrat MPs joined forces with Labour MPs to approve the ban by 376 votes to 107, majority 269.
David Cameron missed the vote while visiting flood-stricken areas in the south west. The prime minister's official spokesman declined to say which way the Prime Minister would have voted had he been able to attend Parliament.
But he told a regular Westminster media briefing: "While he understands the concerns that some have expressed, his view is that the time for this kind of approach has come."
Health Secretary Jeremy Hunt was in favour of the move, together with chancellor George Osborne, chief secretary Danny Alexander, defence secretary Philip Hammond and education secretary Michael Gove, international development secretary Justine Greening, Scotland secretary Alistair Carmichael and energy secretary Ed Davey.
Justice Secretary Chris Grayling was in the "no" camp of those who said it was unenforceable. Home secretary Theresa May, Work and pensions secretary Iain Duncan Smith and Northern Ireland secretary Theresa Villiers also opposed the ban.
A Department of Health spokesman said: "Second-hand smoke is harmful to children and it is right that this has been debated in Parliament. We will now determine how this amendment should be taken forward."