In his speech to the Tory faithful at the Conservative party conference in Manchester in October, Rishi Sunak declared that he wanted to ban smoking for future generations. His announcement – confirming a plan first reported by the Guardian – was met with applause from grassroots members then. But Tuesday night’s vote demonstrates the challenges for a Tory prime minister championing what his own side deems an archetypal “nanny state” policy. The proposed legislation would increase the legal age for buying tobacco products year on year, meaning a child aged 15 or below today would never be allowed to buy cigarettes. Sunak has called it the “biggest public health intervention in a generation” and argued it would save thousands of lives and take significant pressure off the NHS. Health leaders have come out strongly in favour of the policy. But on Tuesday Conservative MPs lined up in the Commons chamber to warn that the proposal was unenforceable, that it would drive illegal tobacco sales and that it was a slippery slope that could lead to bans on fast food or alcohol. A total of 57 Tory MPs dealt a blow to Sunak’s already fragile authority by voting against the bill – and 106 either abstained or were absent. As a result, barely more than half of Conservative MPs voted in favour of the prime minister’s flagship ban. Thanks to Labour’s support, the proposed ban is on course to become law, so this aspect of the PM’s legacy looks secure. But a decision to oppose the ban by Kemi Badenoch, the business secretary who is often touted as a potential leadership challenger to Sunak, will have caused concern in Downing Street. One Tory insider suggested that Badenoch’s decision was “pure politics”. Polling suggests that while a majority of Conservative voters support the ban, a majority of Tory members – those who will decide the results of any future leadership election – do not. A survey by the ConservativeHome website in February suggested that over half of members were against the policy, with only a third supporting it. By declaring she is against the ban, Badenoch – who, according to a survey by the same website this week, is by far and away the most popular cabinet minister among members – gives the Conservative grassroots another reason to vote for her in the event of a leadership contest. Badenoch is not the only prospective leadership contender to have allied herself with critics of the policy. The Commons leader Penny Mordaunt abstained in the vote on Tuesday and indicated she had several concerns about the plan, including its enforceability. Suella Braverman, the former home secretary, and Robert Jenrick, the former immigration minister, also voted against after criticising the bill. Both are said to harbour leadership ambitions. Sunak’s team will be keeping an eye on all this with trepidation as the local elections approach, amid reports that his critics are preparing to launch a leadership challenge. Many Conservative MPs think the idea of changing leaders again is fanciful – but if 53 of them are dissatisfied enough with his leadership to submit a letter of no confidence, it will trigger a vote in the Tory parliamentary party. And if nothing else, the results of Tuesday night’s vote demonstrate that banning smoking is yet another issue which Sunak’s MPs are divided on.