The popular snack has been refused permission to use a symbol showing it is suitable for vegetarians because it included battery-farmed eggs, the Vegetarian Society has said.
However, Masterfoods, which makes Mars, said its own new guidelines on vegetarian labelling conformed with industry guidance on product suitability.
In May the company dropped plans to use animal-based products in its chocolate.
The firm admitted it had made a mistake over its plan to include animal enzyme rennet in its whey after receiving 6,000 consumer complaints.
The Vegetarian Society led the campaign against the move, describing it as "incomprehensible".