The aggressive campaign has seen complaints lodged with authorities including Trading Standards. Adverts placed in British national newspapers last week invited new complainants to come forward.
The campaigners, headed by the US-based Centre for Science in the Public Interest (CSPI), centre on allegations that Quorn products have been responsible for bouts of illness including vomiting and even unconsciousness.
Marlow Foods, which makes Quorn, refuted the allegations, but declined to say if it would be taking action against CSPI.
"Quorn foods have been on sale in the UK for more than 15 years," said a spokesman. "In that time, 20 million consumers throughout Europe and the UK have enjoyed Quorn meals."
He added: "The latest data shows that one in 146,000 Quorn consumers report any suspected adverse reaction."
This compares with 1 in 350 for soy and 1 in 35 for fish and shellfish, making Quorn one of the better-tolerated foods on the market."
CSPI also claims that the products, which hit shelves in the US earlier this year, have been falsely labelled as being mushroom based when, it is claimed, they are actually made from a fungus only distantly related to the mushroom family. The organisations web site reports one American academic as saying the comparison was like "calling a rat a chicken (just) because both are animals".