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Transport chiefs have changed their “confusing” junk food advertising rules — after even one of their own adverts, featuring strawberries and cream, fell foul of the guidelines.

TfL bosses have made the changes just four months after they launched the ban, the Standard can reveal.

Mayor Sadiq Khan outlawed adverts showing food and drinks with high fat, salt and sugar on the Tube, Overground and bus network in February. He said he wanted to tackle the “ticking time-bomb” of child obesity in the capital.

But six of the 10 advertising policy rules have been revised, after a number of controversial decisions.

TfL allowed a bucket of KFC to be promoted but rejected an advert for Farmdrop, which delivers fresh produce across London. TfL decided to remove strawberries and cream from its own Wimbledon advert because it thought it could contravene the rules by promoting a fatty food.

Now the edited rules clarify that when such a food is “incidentally” featured, TfL can permit the advert if it is not deemed to be promoting that product.

A further change will clamp down on brands using children to advertise products while another says advertisers must be clear about portion sizes.

This means a bucket of KFC will have to be labelled as a serving for a family. TfL’s head of commercial media Chris Reader has written to advertisers in the past few days, telling them: “We have decided to update our guidance document.”

It is estimated the junk food advert ban, backed by celebrity chefs Jamie Oliver and Hugh Fearnley-Whittingstall, will cost TfL £13 million a year in lost revenue. Some advertising experts predict it could eventually cost up to £35 million a year. TfL is giving foods a score based on the balance between beneficial nutrients and the ingredients which children should reduce in their diet.

Conservative London Assembly member Andrew Boff said: “Sadiq Khan was warned his junk food ad ban was rushed and confusing. The Mayor ignored those warnings and as a result his policy has turned into a shambolic mess.” Outsmart, the outdoor advertising trade body, said the changes “show how difficult it is to apply a ‘one size fits all’ approach … the Mayor’s ban is an ongoing publicity stunt”.

TfL insists that it has not “changed” the policy but it has “amended” some of the rules.

A TfL spokesperson said: “Child obesity in London is a serious issue and the Mayor is determined to do all he can to tackle it. Our policy,which has not changed since we introduced it in February, is designed to reduce children’s exposure to adverts for food and drink which could contribute to this problem.