The Food Standards Agency (FSA) has said it is investigating allegations that traces of meat were found in vegetarian and vegan ready meals.
Laboratory tests reportedly found traces of pork in Sainsbury's "meat free" meatballs and traces of turkey in avegan macaroni ready meal from Tesco, the Daily Telegraph has reported.
An FSA spokeswoman said: "Our priority is to ensure consumers can be confident that the food they eat is safe and is what it says it is.
"We are investigating the circumstances surrounding these alleged incidents and any resulting action will depend upon the evidence found."
The Sainsbury's item has been approved by the Vegetarian Society, which means it has been independently checked.
Both supermarkets said their own tests have not found any meat traces.
The Telegraph said it sent a number of samples to a German Government accredited food testing laboratory.
The results reportedly showed traces of pork DNA in Sainsbury's own brand Meat Free Meatballs 380g, which retail at £1.50.
It also allegedly found traces of turkey DNA in Tesco's Wicked Kitchen BBQ Butternut Mac 385g ready meal, which is labelled as being vegan and retails at £4.
The presence of whole animal DNA indicates that a dish contains meat or animal skin, the laboratory told the newspaper.
A Sainsbury's spokeswoman said: "These products are produced at a meat-free factory.
"Sainsbury's and the Vegetarian Society also carry out regular checks and no issues have been found.
"We are concerned by these findings however and are carrying out a comprehensive investigation alongside our supplier."
A Tesco spokesman said: "We take the quality and integrity of our products extremely seriously and understand that our vegan and vegetarian products should be exactly that.
"Our initial DNA tests have found no traces of animal DNA in the BBQ Butternut Mac product available in stores today.
"We would urge the Telegraph to share full details of their testing, including the lab used as we continue to investigate."