The Mars bar cannot use a symbol showing it is suitable for vegetarians because it includes battery-farmed eggs, the Vegetarian Society has said.
New vegetarian guidelines will appear on the back of bars
The confectionery missed the society's accreditation standards, allowing it to carry its respected trademark symbol.
Masterfoods, which makes Mars, said its own new guidelines on vegetarian labelling conformed with industry guidance on product suitability.
In May Masterfoods 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".
Mars subsequently published what it said were "clear" labelling advice for vegetarians, in line with guidance from the Food Standards Agency (FSA).
But the society said Mars bars could not qualify for its 46-year old seedling symbol, which it describes as the "gold standard of vegetarian suitability", because the eggs they contain are not free range.
Although many vegetarians eat dairy products, the organisation said its members would expect any eggs used in accredited products to be free range and would be "horrified" to discover otherwise.
"We are pleased that Masterfoods has recognised the importance of labelling to its vegetarian customers," the society's chief executive Annette Pinner said.
"However, it is important for consumers to recognise the difference between minimum vegetarian standards and the higher criteria associated with Vegetarian Society approval."
Mars said its new labelling scheme would ensure the suitability of its products was "clearly communicated" to vegetarians.
"We believe introducing our scheme based on FSA guidance provides us with the best solution as the FSA is a respected independent authority," said a company spokesperson.
"We understand the Vegetarian Society worked with the FSA in developing the guidance."