A vegan group's "misleading" advert linking cow's milk to cancer has been banned.
The poster by activists Viva! triggered two complaints to watchdogs after being seen on buses in Bristol last September.
The ad was banned following an investigation by the Advertising Standards Authority (ASA) because Viva! was unable to substantiate claims that hormones in cow's milk are linked to cancer.
The ad featured an image of a cow's udder and included the claims "Some dairy industry facts we bet you don't know … Most cows are pregnant when milking. That's why milk contains 35 hormones, including oestrogen … some of these are linked to cancer. Milk is for babies, so let Viva! wean you off the teat!"
Two people complained to the ASA that the ad's claim "milk contains 35 hormones, including oestrogen … some of these are linked to cancer" implied that drinking cow's milk could cause cancer, was misleading and couldn't be substantiated.
Viva! said the claim referred to hormones that were naturally occurring in cow's milk, which increased during pregnancy and were essential for calf development.
The campaign group said the claim did not refer to hormone treatments or artificial hormones.
Viva! highlighted that the ad stated the hormones were "linked to" cancer, rather than that they "caused" cancer.
In support of the claim, they referred to a range of papers, which they believed showed the presence of more than 35 hormones in cow's milk, including oestrogen hormones and insulin-like growth factor (IGF-1).
But the ASA found that the poster breached rules regarding misleading advertising and substantiation, and banned it from appearing again.
An ASA spokesman said: "While the claim stated that some hormones in cow's milk were "linked" to cancer rather than definitively stating that they caused cancer, we considered that consumers would nonetheless interpret it to mean that because of the hormones that were present in cow's milk, drinking cow's milk could increase a person's risk of developing cancer.
"We considered that the various sources provided by Viva! constituted adequate evidence that over 35 hormones were present in cow's milk.
"However, we were concerned that the studies were unable to account for confounding factors which could affect the results.
"We also noted that the papers referred to other conflicting evidence and all noted the need for additional studies to confirm their findings.
"The studies did not support Viva's assertion that the findings of increased risk of cancer were specifically a result of the hormones present in cow's milk rather than to other factors.
"We therefore concluded the claim 'milk contains 35 hormones, including oestrogen …some of these are linked to cancer', as it would be understood by consumers to mean that due to the presence of hormones, drinking cow's milk could increase a person's risk of developing cancer, had not been substantiated and was therefore misleading."
He added: "The ad must not appear again in the form complained about.
"We told Viva! not to make claims which stated or implied that due to the presence of hormones, drinking cow's milk could increase a person's risk of developing cancer."
Viva! founder and director Juliet Gellatley said: "There's plenty of scientific data linking milk and other dairy products to an increased risk of some cancers and many researchers are pointing the finger of blame at the hormones naturally present in dairy."