The mayor also proposed to prohibit the opening of new outlets selling fast foods closer than 400 metres from schools.
Sadiq Khan has published proposals which mean posters for food and drink which is high in fat, salt or sugar would be banned from 16,000 advertising spots under draft plans announced by Transport for London (TfL).
The Mayor said: "Child obesity in London is a ticking timebomb and I am determined to act". He recently appointed a chair and vice chair of London's first Child Obesity Taskforce, which will meet for the first time later this year.
TfL's director of TfL's transport strategy Lilli Matson said it had a "large advertising estate with a diverse audience", and is supporting the mayor's attempts to make London healthier.
Khan said: "I'm delighted that so many Londoners have got involved and given their feedback on the future of London's environment".
The chef and health campaigner said the proposal, which aims to tackle childhood obesity, would send a "massive message".More news: German Diplomat Vows To Preserve Iranian Deal
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Mr Khan wants to "reduce the influence and pressure that can be put on children and families to make unhealthy choices".
"We recognise the major challenge that obesity presents".
The chef and campaigner said: 'This is a game-changing moment, protecting kids from relentless junk food advertising on their daily journeys to school and around our incredible city'.
Doctors say advertising is one of the leading causes of the problem especially in big cities where there is also a high level of deprivation.
'We know healthy children are much more likely to develop into healthy adults and with the bill for obesity weighing in at just over £5 billion, it is clear that action is needed if we are to reverse this trend.
"Children are inundated with adverts for unhealthy food so this is a really encouraging move and a bold step in the right direction".