The government has written to Sainsbury's and Morrisons asking them to stop "advertising and promoting" heated tobacco products, which it says is against the law.
The BBC reported in February the supermarkets were displaying posters and video screens showing devices which create a nicotine-containing vapour by heating tobacco with an electric current.
At the time, both supermarkets said they believed the adverts were legal.
In response to the letter, Sainsbury's said it was in "close contact with the government", while Morrisons said it would reply "in due course".
In 2002, the Labour government under Tony Blair passed a law banning tobacco advertising. It defined a tobacco product as something designed to be "smoked, sniffed, sucked or chewed".
Morrisons has argued that this means that it doesn't apply to heated tobacco products, as they don't produce smoke.
Advertising for Philip Morris International's (PMI) iQos heated tobacco device on posters and video screens was still on display in Sainsbury's and Morrisons stores visited by the BBC in June, where they were visible to children.
PMI said it believes the Department of Health's interpretation of the law is wrong, and said it has "complied with all applicable laws and regulations" since it launched iQos in 2016.
The government has now written to the supermarkets clarifying that in its opinion, the law does apply to these products.
A Department of Health and Social Care (DHSC) spokesperson told the BBC: "In May, we wrote to supermarkets reiterating that the Tobacco Advertising and Promotion Act 2002… applies to all tobacco products currently on the market, and formally requested they stop advertising and promoting heated tobacco products in stores.
"All tobacco products are harmful to health," the spokesperson added.
Surveys by the charity Action on Smoking and Health suggest that awareness of heated tobacco products has risen sharply over the past year, and is even higher among young adults, compared with those over 40.
Among 11 to 17-year-olds, nearly a quarter had heard of heated tobacco, up from 7.1% in 2022, the last time they were surveyed.
Some 3.3% of respondents to their survey said they had tried heated tobacco, and for 11 to 17-year-olds, the figure was 2.7%. While low, the charity said this was still "worryingly similar to the levels of use among adults".
Experts say that although research on the health effects of heated tobacco is limited, it is likely to be less harmful than cigarettes, but worse for you than vapes, and less effective at helping smokers quit.
A spokesperson for Morrisons said it was reviewing the letter and would respond "in due course".
Sainsbury's said it believed its ads were compliant with the law. A spokesperson said: "We remain in close contact with the government and industry partners and are planning our transition to ensure we also comply with planned incoming legislation."
It would be for a court to rule definitively whether the government is right that heated tobacco advertising is banned under current law - but so far no-one has brought a case.
The law will be clarified when the government passes the Tobacco and Vapes Bill, which is expected to conclusively ban all tobacco and vape advertising and sponsorship.
The bill is making its way through parliament and is currently at the committee stage in the House of Lords.
Hazel Cheeseman, the chief executive of Action on Smoking and Health, urged the government to pass the law as quickly as possible.
"It is outrageous that certain supermarkets still do not seem to be prepared to comply with the law, even when told they are in breach.
"The longer this takes to resolve, the more children will be exposed to tobacco product marketing," she added.
The Tobacco Advertising and Promotion Act applies UK-wide, but health is a devolved issue. The devolved administrations in Northern Ireland, Wales and Scotland all said they agreed with the DHSC in England that advertising heated tobacco is banned.
Asda and Tesco both said they do not accept tobacco advertising.
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