The poster imposed recurring text saying 'E Coli, Mad Cow, Swine Flu, MRSA' on the words 'Meat Kills' in large capital letters. Swine flu was more prominent in the text than the other infections.
The ASA concluded that some readers might think the advert meant that eating meat caused swine flu. The message was "at best ambiguous" and could cause undue fear and distress, it said.
Peta responded by saying its intention was to highlight the role of livestock production in the incubation and spreading of infectious diseases such as swine flu.
In a separate adjudication, the ASA rejected a complaint against a TV advert that ran as part of the NSPCC's Full Stop campaign and included the claim "together we can stop child abuse".
A lecturer in childhood studies argued that the advert was misleading because charitable work could reduce abuse but not stop it. But the watchdog said viewers were unlikely to believe from the advert that their donation or a single charity could stop abuse entirely.