Chipping Barnet MP Theresa Villiers voted against a ban on smoking in cars carrying children because “the case wasn’t made for it”.
The Conservative politician was among the minority of Tories who opposed plans to introduce the legislation at a Commons vote on Monday.
The motion to amend the Children and Families Bill was approved by a majority of 126 Tories, plus all 202 Labour members and 41 out of 45 Liberal Democrats present.
Ministers now have the power, but are not compelled to, introduce the ban in England.
Opponents to the ammendment say the law would be impossible to enforce and is a move too far towards a nanny state.
Ms Villiers was among 100 Conservatives who voted against the law change during the ballot, which was won by 375 votes to 106.
The Secretary of State for Northern Ireland said: “I considered the pros and cons of a ban carefully. On balance, I decided that the case wasn't made for a ban. I was concerned about how a ban would be enforced.
“I also felt it was difficult to distinguish between smoking in cars when children were present and smoking in the home.
“I believe that politicians shouldn't ban things unless there is a very clear and compelling case to do so. I didn't feel that there was in this instance so I voted against."
Finchley and Golders Green MP Mike Freer voted in favour of the legislation, while Hendon MP Matthew Offord was absent from the vote.