Kemi Badenoch and other senior Tories voted against Rishi Sunak’s phased smoking ban in a bodyblow to the prime minister’s authority.

The cabinet minister said before the vote she could not support the Tobacco and Vapes Bill because she believes in the “principle of equality under the law”.

MPs voted in favour of the bill by 383 to 67 at its second reading in parliament. Some 59 Conservatives voted again the legislation, but Labour support means the bill is likely to sail through parliament.

The PM’s plan will make it illegal to sell tobacco products to anyone currently 15 or younger. The legal age for buying tobacco – currently 18 – would increase every year by one year so that people born in or after 2009 will never legally be able to buy cigarettes.

Tory MPs have been granted a free vote, meaning they can vote with their personal conscience rather than follow the official party line, giving libertarian-minded Conservatives the chance to register their unease without consequence.

Badenoch’s rebellion is significant as she is seen as a future leader of the Conservative Party, with some suggesting she would be in the running to take over from Sunak if he was ousted even before the election.

Former home secretary Suella Braverman and ex-immigration minister Robert Jenrick – both also touted as potential leadership contenders – also voted against the bill.

Badenoch, the business and trade secretary, wrote on X: “I’m not a smoker and think it is an unpleasant habit, costly for both the individual and society. The PM’s intentions with this bill are honest and mark him out as a leader who doesn’t duck the thorny issues.

“I agree with his policy intentions BUT I have significant concerns and appreciate the PM making this a free vote. It gives me the opportunity to express my personal view, outside collective responsibility. The principle of equality under the law is a fundamental one. It underpins many of my personal beliefs.

“We should not treat legally competent adults differently in this way, where people born a day apart will have permanently different rights. Among other reasons it will create difficulties with enforcement. This burden will fall not on the state but on private businesses.

“Smoking rates are already declining significantly in the UK and I think there is more we can do to stop children taking up the habit. However, I do not support the approach this bill is taking and so will be voting against it.”

The prime minister announced the plan at the Conservative Party conference in October.

Sunak said the change would “save more lives than any other decision we could take” as 64,000 people a year currently die from smoking.

The bill will also give the government new powers to clamp down on young people vaping, which include imposing restrictions on flavours and regulating the way they are packaged and sold to make them less appealing to children.