The British Heart Foundation's director of policy and communications, Betty McBride, said: "Tobacco advertising is rightly banned in the UK, yet current glitzy packaging clearly still advertises tobacco on the cigarette box.
"It's an absurd loophole the tobacco industry takes full advantage of to lure in new young smokers."
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The suspect, 35-year-old Willie Corey Godbolt, has been arrested and was being treated for a gunshot wound in hospital.
Mohanna Abdhou, known as Montana, was out with friends when two attackers cycled up to them and opened fire in Brent, north-west London.
The US president tweeted that Mrs May gave him "full details" of the incident during talks while he attended summits in Europe.