A ban on junk food advertising across London's entire public transport network will be introduced next year. Under the scheme, posters for food and drink high in fat, salt and sugar will vanish from the Underground, Overground, buses and bus shelters. London Mayor Sadiq Khan has said he wants to tackle the "ticking time bomb" of child obesity in the capital. The Advertising Association has said it would have "little impact on the wider societal issues that drive obesity". The ban will take effect across the Transport for London (TfL) network on 25 February.The scheme is backed by child health experts including chief medical officer for England Professor Dame Sally Davies who described it as an "important step in the right direction". However, the Advertising Association said commuters could suffer as a result of the ban. Chief Executive Stephen Woodford said the UK already has "the strictest rules in the world when it comes to advertising high fat salt sugar foods", which mean under-16s cannot be targeted. "This will lose revenue from advertising for TfL and that will potentially have an impact on the fares that passengers have to pay," he said.