A vegan customer is suing Burger King for cooking its plant-based patties on the same grills it uses for meat.
In a proposed class action filed in the US, Philip Williams said the way the Impossible Whopper is grilled leaves it "coated in meat by-products".
He said the burger's tagline - "100% Whopper, 0% Beef" - was misleading.
Burger King did not comment, but the small print on its website says people wanting a meat-free option can request "a non-broiler method of preparation".
A spokeswoman for the supplier, Impossible Foods, also told Reuters news agency that vegetarians and vegans "are welcome to ask" for their Impossible Whopper to be cooked in a microwave.
In the lawsuit filed in a Miami federal court, Mr Williams says that the burger chain does not clearly advertise that the plant-based burgers 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 was he told the Whopper was cooked on the same grill as the meat burgers, he said - adding that, had he known, he would not have ordered it.
Mr Williams said he wanted damages for everyone in the US who bought the Impossible Whopper, as well as an injunction requiring Burger King to "plainly disclose" that the vegan burgers and meat burgers are cooked on the same grills.
Burger King started selling the Impossible Whopper in Sweden in May, before rolling it out to US stores in August. It started selling the meatless burger in 25 other European countries earlier this month.
According to Burger King's suggested pricing, the plant-based burger costs US customers about a dollar more than the beef version.