(Bloomberg) -- Rishi Sunak faces a backlash from Conservative lawmakers in a vote to ban smoking for future generations, further exposing the divisions undermining his leadership.

Members of Parliament will deliver their verdict later Tuesday on legislation that would gradually raise the legal age for buying cigarettes by one year annually, so that any 14-year-old today would never legally be able to smoke. If it becomes law, the UK would become the first country to implement such an approach, after New Zealand backtracked on a similar plan.

Passing the law would be a “major public health achievement,” Chris Whitty, England’s chief medical officer who became a household name during the pandemic, told BBC radio. “We do expect that — over time — to lead to essentially smoking dying out almost completely.”

But while the government has offered Tory MPs a “free vote,” meaning any who go against Sunak would not be seen as rebels, the action in Parliament is still hazardous for the prime minister. Government whips expect more than 50 Tory MPs to oppose the legislation, perhaps including Business Secretary Kemi Badenoch and other prominent Tories seen as potential future party leaders.

The issue is not whether the bill will pass, because Keir Starmer’s poll-leading opposition Labour Party has said it will vote with Sunak to get the legislation through. Instead, the risk for Sunak is that the vote underscores the depth of internal opposition to his approach.

Sunak finds himself in an uncomfortable position within his own party in an election year. Labour leads the Tories by around 20 points in opinion polls and is widely expected to win power at a public vote that must take place in the next 10 months. Some Tory rebels have been agitating for a change of leader after May 2 local elections to try to improve their party’s standing in the polls.

The smoking ban has been seized on by Sunak’s Tory critics on the libertarian right, such as former premier Liz Truss, who called it “unconservative” in an interview with the BBC on Monday. Truss’s predecessor Boris Johnson also hit out at the plan, calling it “absolutely nuts” at an event in Canada last week. The policy was one of Sunak’s flagship announcements at the Conservative Party conference last year and is viewed as one of his key legacy priorities.

Some Conservatives have indicated their unease at what they see as a tendency by Sunak’s administration to ban perceived societal ills. The premier has also proposed banning disposable vapes and is considering a crack down on social media use by teenagers.

Yet the smoking ban has broad support among voters, including in seats likely to be the battleground in the general election. In political terms, Sunak’s Tory opponents are effectively arguing against him on ideological grounds, even though the polls suggest his plan will boost the Conservative Party’s chances.

A YouGov survey in October found 62% of Britons back the policy. Research by the More in Common think tank found voters for all four main UK political parties, including the right-wing Reform UK, are more likely to support the ban.

Other Tories support the phasing out of smoking on health grounds. Home Office minister Laura Farris told broadcasters on Tuesday that smoking was responsible for one in four deaths in the UK and cost the National Health Service £17 billion a year, adding that she was “not particularly interested in arguments about freedom on this one.”

Jesse Norman, a former minister, defended the plan by arguing it “protects young people from a dangerous addictive drug, supports the NHS and saves the taxpayer and society a fortune.”

Resistance to the ban from libertarians is “surprising,” Whitty said. “The great majority of smokers wish they had never started, but they become addicted at an early age and then they’re trapped and their choice has been taken away.”

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