The prime minister said the proposed legislation would mean a "14-year-old today will never legally be sold a cigarette and that they and their generation can grow up smoke-free".

He said the changes will be subject to a vote in parliament but this will be a free vote, as was the ban on smoking in public places and raising the smoking age to 18.

Read more: See all the key points from Sunak's conference speech

"There will be no government whip. It is a matter of conscience," he said.

Addressing the annual Tory party conference today, Mr Sunak also promised to restrict the availability of vapes under plans to "put the next generation first".

On smoking, he said it would not be fair "to take away the rights of anyone to smoke who currently does".

However, he said more could be done to stop teenagers taking up cigarettes in the first place.

"I propose that in future we raise the smoking age by one year every year," he told party delegates in Manchester.

"That means a 14-year-old today will never legally be sold a cigarette and that they and their generation can grow up smoke free."

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He added: "People take up cigarettes when they're young - four in five smokers have started by the time they're 20.

"Later the vast majority try to quit. But many fail because they're addicted and they wish they had never taken up the habit in the first place.

"And if we could break that cycle, if we could stop the start, then we would be on our way to ending the biggest cause of preventable death and disease in our country."

Downing Street said it expects that the plans will mean up to 1.7 million fewer people smoking by 2075.

On vapes, the prime minister said: "As any parent or teacher knows, one of the most worrying trends right now is the rise in vaping amongst children - one in five children have used vapes.

"We must act before it becomes endemic.

"So we will also bring forward measures to restrict the availability of vapes to our children, looking at flavours, packaging displays and disposable vapes."

Ministers have faced repeated calls to ban vapes to help protect children and reduce the significant environmental impact of the single-use products.

The government set out an ambition in 2019 for England to be smoke free by 2030.

It commissioned a review, published last June and led by Dr Javed Khan, which made a series of recommendations, including increasing the legal age for buying tobacco.

He recommended that the age of sale should increase from 18, by one year every year, until no-one can buy a tobacco product.

Smoking causes around one in five cancer cases and more than one in four cancer deaths each year in the UK.

Almost six million people in England still smoke.

Dr Khan put the annual cost to society of smoking at around £17bn, with a cost of £2.4bn each year to the NHS alone.

Cancer Research UK's chief executive Michelle Mitchell said: "Raising the age of sale on tobacco products is a critical step on the road to creating the first ever smoke-free generation."

But Simon Clark, director of the smokers' group Forest, said: "Raising the age of sale of tobacco is creeping prohibition, but it won't stop young people smoking because prohibition doesn't work.

"Anyone who wants to smoke will buy tobacco abroad or from illicit sources.

"Future generations of adults who are considered old enough to vote, pay taxes, drive a car and drink alcohol are going to be treated like children and denied the right to buy a product that can be purchased legally by people a year older than them."