Tobacco firms are lobbying MPs and peers in an effort to derail Rishi Sunak’s flagship policy to phase out smoking, the head of Britain’s biggest cancer charity has said. The prime minister’s landmark legislation – which would bar anyone born after 2009 from buying cigarettes and make England the first country in the world to ban smoking – is due to be debated in parliament for the first time on Tuesday. Michelle Mitchell, the chief executive of Cancer Research UK, said the tobacco industry was working behind the scenes using “a variety of tactics” to try to weaken, delay or even kill off the plans. The government’s tobacco and vapes bill would prevent anyone who is turning 15 this year, or younger, from ever being able to legally buy tobacco products. Mitchell told the Guardian that the “world-leading” legislation was “the most important public health policy shift I can remember” and could “see the blight of tobacco removed” from society for ever. Smoking is the single biggest cause of cancer in the UK and worldwide and causes at least 15 different types of the disease, she said. But the proposals to bring in a smoking ban for the next generation were being privately undermined by tobacco companies. “We know the tobacco industry is working quite hard to dilute the bill,” Mitchell said. “MPs and peers have briefed us that members of the tobacco industry are seeking to make arguments [against] and amendments to the bill as it goes through the passage of parliament.” The tobacco industry is lobbying MPs and peers to oppose the legislation and seeking support for instead raising the smoking age from 18 to 21 in an attempt to avoid an outright ban on buying cigarettes for anyone who turns 15 this year, Mitchell said. She said representatives of tobacco companies were also trying to persuade politicians to back exemptions, “for example excluding cigars”, from the legislation. Separate efforts were being made to delay the passage of the bill until after the general election. Another tactic was to promote the idea of a clause in the bill to guarantee a review of the legislation in future. The danger is that in theory this could lead to the smoking ban being overturned, Mitchell said. “The tobacco industry in the UK and around the world uses the same tactics to resist, stop, postpone any pieces of legislation which have a net negative impact on their business,” she said. “It’s really vital that MPs and peers don’t get distracted by the noise, not least from the tobacco industry – and really focus on the huge public health benefit that would come from this.” Similar legislation had been due to come into effect in New Zealand, but this was repealed by the country’s new coalition government in February. Previously due to take effect from July, the toughest anti-tobacco rules in the world would have banned sales to people born after 2009, cut nicotine content in smoked tobacco products and reduced the number of tobacco retailers by more than 90%. Researchers and campaigners warned the policy reversal would mean people could die as a result. Deborah Arnott, the chief executive of Action on Smoking and Health (Ash), said she was not surprised UK MPs and peers were being lobbied over the legislation, particularly given the global impact it could have on the tobacco industry. “The tobacco transnationals will fight tooth and nail to block, water down or at the very least delay the UK’s historic legislation to raise the age of sale, because it is an existential threat to their business model,” she said. “They may claim they want a smoke-free future, but most of their sales and even more of their profits still come from selling cigarettes, which are sold for vast amounts more than the pennies they cost to make. “The lesson of all previous tobacco laws is that once they come into force in one country they spread rapidly round the world. That’s what happened with ad bans, smoke-free laws and plain packs, and that’s why big tobacco can’t afford to let this legislation pass unchallenged.” Last week Boris Johnson attacked Sunak’s smoking ban plan as “absolutely nuts”. Speaking at a gathering of conservatives in Ottawa, Canada, the former prime minister said: “When the party of Winston Churchill wants to ban cigars, donnez-moi un break as they say in Quebec, it’s just mad.” According to documents seen by the Guardian, among those with hospitality suites at the Canada Strong and Free networking conference were some of the world’s largest tobacco companies.