The Vegetarian Society (VS) has been warned its endorsement of McDonalds' meat-free foods will upset vegetarians.
McDonalds' Quorn burgers, salads and fruit have been endorsed
Quorn burgers, fruit toast and bagels are now on the menu at the fast food chain's UK outlets - marked with the society's "approved" logo.
In response The Vegan Society (TVS) has said it believes many vegetarians will be against the move.
The Manchester-based VS said the aim of the product-approval is to "bring vegetarianism into the mainstream."
"It's when you add 'approved by' to a product, it's like condoning it along with everything else," said Tony Bishop-Weston, of the TVS .
"I think there's going to be a lot of unhappy vegetarians and vegans out there, because they [McDonalds] are like the anti-Christ to them."
However, the fact McDonalds is spending its money promoting vegetarianism "can't be that bad", he explained.
"[But] I doubt very much it'll be vegetarians and vegans that'll be going in there, it'll be meat eaters who'll start eating more vegetarian food," he added.
The VS said it is an educational charity, not an animal rights pressure group.
In a statement it said: "Vegetarian Society approved products are carefully checked before being licensed.
"Every single ingredient and process involved in both the factory environment and restaurant kitchen has been scrutinised to ensure UK consumers can order a vegetarian meal in McDonalds, safe in the knowledge that its ingredients are truly free from slaughter products."
Andrew Butler, from People for the Ethical Treatment of Animals, said it approves of fast food endorsement, having backed the BK Veggie, a vegan burger in Burger King's US outlets.
"Fast food restaurants need to be encouraged to make a move towards more ethical and healthier food choices," said Mr Butler.