Burger King has been banned from advertising its Rebel Whopper as "100% plant based" as it is cooked on the same grill as meat products.
The Advertising Standards Agency (ASA) found Burger King had also failed to make it immediately clear its new product was dressed with an egg-based mayonnaise.
It received complaints from consumers criticising the use of the claims "100% Whopper. No Beef" and "plant-based burger" in Burger King's marketing.
The watchdog said in a ruling: "Because the overall impression of the ads was that the burger was suitable for vegans and vegetarians when in fact it was not, we concluded that the ads were misleading."
The ASA found the product's launch had been timed to coincide with the "Veganuary" challenge – making consumers believe it was fully vegan.
The decision was based on three adverts across Twitter and Facebook.
A tweet from the @BurgerKingUK account said: "You asked and we listened. Introducing the Rebel Whopper, our first plant-based burger!"
The tweet was accompanied by an image of the burger stating "100% Whopper. No beef".
An almost identical advert appeared on Facebook with the additional claim "Powered by the vegetarian butcher" and in small print at the foot "*Product is cooked alongside meat products".
A second Facebook ad stated the Rebel Whopper "Tastes of being woke", had below it the claim "100% whopper. No Beef", and included the Burger King logo and The Vegetarian Butcher logo
In small print the ad had "T&C's apply".
BKUK Group Ltd, the chain's parent company, said the small print indicated the burger may not be suitable for vegans or vegetarians, and that this had been communicated to journalists, on social media posts and in dialogue with customers.
It said the product itself consisted of a 100% plant-based patty supplied by the Vegetarian Butcher and that a customer who did not want mayonnaise could exclude it from their order.
The ASA found the qualification that the burger was "cooked alongside meat products" was not sufficiently prominent to counter the impression that the burger was suitable for vegetarians and vegans.
It added the small print did not refer to the presence of egg mayonnaise and was missing from one Facebook advert completely.
It ruled the advert must not appear in its current form again.
Last year, Burger King was pulled up by the ASA for apparently encouraging customers to "milkshake" politicians.
An Edinburgh McDonald's outlet said it would not be selling milkshakes during Brexit party leader Nigel Farage's visit for a political rally.
The Burger King account tweeted: "Dear people of Scotland. We're selling milkshakes all weekend. Have fun. Love BK. #justsaying."
The ASA said it had told Burger King the ad must not appear again and "to ensure that its future marketing communications did not condone or encourage anti-social behaviour."