The mayor also suggested a ban on new hot-food takeaways opening within 400 metres (yards) of schools, his office announced.
“Child obesity in London is a ticking timebomb and I am determined to act,” Khan said.
“It can’t be right that in a city as prosperous as London that where you live and the income you have can have a massive impact on whether you have access to healthy, nutritious food and your exposure to junk food advertising.
“That is why I’m proposing to ban adverts for harmful junk food from our entire Tube and bus network.”
Almost 40 percent of children in London aged 10 and 11 are overweight or obese, according to official data.
Celebrity chef Jamie Oliver called the plans “a massive and bold step forward”.
Lobby group the Advertising Association pointed out that Britain already bans the advertising of high fat, sugar and salt content food and drink products in media where children make up more than 25 percent of the audience.
“International experience and independent research have shown an advertising ban would have little impact on the wider societal issues that drive obesity,” a spokesman for the group told the BBC. DM
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