The Advertising Standards Authority (ASA) said the chain’s claim that the burger is “100% Whopper, no beef” could be understood to mean it did not contain animal products.
Burger King said it had been “clear and transparent” in its marketing.
The Vegan Society said it was a “missed opportunity”.
“We communicated from the outset that the Rebel Whopper is aimed at a flexitarian audience,” the fast food chain said in a statement.
But the ASA found that Burger King’s social media posts about the Rebel Whopper gave the impression it could be eaten by vegans and vegetarians.