An ad by veganism activist group Viva! has been banned for including "misleading" claims that hormones in cows' milk are linked to cancer.
The poster, displayed on buses in September last year, 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."
Two people complained that the poster's implication that drinking cows' milk could cause cancer was misleading.
Viva! highlighted that the ad stated the hormones were "linked to" rather than caused cancer.
It said the wording was commonly used to express an association when there was a potential or likely relationship but not an absolute causative relationship, and consumers were used to seeing the phrase in that context.
The group referred to a range of papers which it believed demonstrated the presence of more than 35 hormones in cows' milk, including oestrogen hormones and insulin-like growth factor (IGF-1), and a further seven papers which it believed provided sufficient evidence to support the claim that some of the hormones in cows' milk were linked to cancer.
The Advertising Standards Authority (ASA) said consumers would interpret the ad to mean that drinking cows' milk could increase a person's risk of developing cancer.
The ASA said the various sources provided by Viva! provided adequate evidence that over 35 hormones were present in cows' milk.
Considering whether the papers were sufficient evidence for the claim that some of those hormones were linked to cancer, the ASA concluded that they were of good methodology and had statistically significant findings, but said the papers referred to other conflicting evidence and all noted the need for additional studies to confirm their findings.
The ASA said: "The studies and meta-analysis did not support Viva's assertion that the findings of increased risk of cancer were specifically a result of the hormones present in cows' 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 cows' milk could increase a person's risk of developing cancer, had not been substantiated and was therefore misleading."
It ruled that the ad must not appear again and told Viva! "not to make claims which stated or implied that due to the presence of hormones, drinking cows' 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."