A vegan customer is suing Burger King for cooking his herbal patties on the same grill he uses for meat.
In a US class action lawsuit, Philip Williams said the way the Impossible Whopper is grilled makes it "coated with meat by-products."
He said the hamburger slogan – "100% Whopper, 0% Beef" – was misleading.
Burger King has not commented, but the small print on its website says that people who want a meatless option can request "a non-chicken preparation method."
A spokeswoman for the supplier, Impossible Foods, also told Reuters news that vegetarians and vegans "can order" their Impossible Whopper to be microwaved.
In a lawsuit filed in a Miami federal court, Williams says the hamburger chain does not clearly announce that herbal hamburgers are cooked with meat.
He said he visited a drive-through restaurant in Atlanta, Georgia, and ordered the Impossible Whopper without mayonnaise.
At no point did he say Whopper was cooked on the same grill as beef burgers, he said – adding that if he had known he would not have ordered it.
Williams said he wanted damage for every US who bought Impossible Whopper, as well as an injunction requiring Burger King to "clearly disclose" that vegan burgers and beef burgers are cooked on the same grills.
Burger King started selling Impossible Whopper in Sweden in May, before launching it in US stores in August. It started selling the meatless hamburger in 25 other European countries earlier this month.
At Burger King's suggested prices, the herbal burger costs US customers about a dollar more than the beef version.